Sometimes, The Diplomat ends in booming explosions, like with the Season 1 finale. But, the drama’s Season 3 finale (now streaming) reveals that quieter moments can also be quite devastating. In the last seconds of Episode 8, diplomat Kate Wyler (Keri Russell) fears she may have been double-crossed by her own husband, Vice President Hal Wyler (Rufus Sewell). And, his accomplice is none other than the President of the United States, Grace Penn (Allison Janney). 

“We talk about Season 3 as ‘the split,’” The Diplomat creator Debora Cahn tells Tudum. “It’s the time when Kate and Hal’s relationship really falls apart. It’s the time when the US-UK relationship really falls apart.” 

Hal and Grace’s latest apparent scheme now threatens to blow up what is left of the American-British partnership. After speaking with spy Callum Ellis (Aidan Turner) and Grace’s husband Todd Penn (Bradley Whitford), Kate comes to question whether Hal and Grace stole the Poseidon missile, a Russian nuclear weapon of “doomsday”-level destruction. Kate knows such a move by the American president and vice president would be perceived as an “act of war” by British prime minister Nicol Trowbridge (Rory Kinnear), who is already wary of American overreach. Just how alarmed should viewers be about what’s next? 

“They should be worried. There’s plenty to worry about,” Cahn says. Kate, certainly, would agree. She ends Season 3 watching Grace and Hal pose for photos — each image more menacing than the last. “It can be pretty chilling when you see who takes power and get the sense that they may be horrifically misusing it,” Cahn says. 

So how did the Wylers end up in this catastrophic situation? What is the Poseidon missile, and how could it have wound up in American hands? And, how did Hal even become the vice president? Keep reading as Cahn answers all those questions and more about The Diplomat Season 3 — and what it all means for Season 4, which has already been announced.  

Why is Hal Wyler the Vice President? 

Hal only knows how to fail up. At the end of Season 2, his actions accidentally hasten the death of President Rayburn (Michael McKean). But, at the close of the Season 3 premiere, Hal is offered the office of the vice president by the new POTUS, Grace Penn. 

On the one hand, “Hal feels like he killed Rayburn. He’s gripped with this sense of horror at the chain of events that he kicked off,” Cahn says. “That said, Hal is a guy who moves forward. He lives like a goldfish.”

In Episodes 2 and 3, Hal fights for the VP spot. By the Season 3 finale, Hal and Grace are an unstoppable political force (who may have stolen a nuclear weapon, but more on that later). Cahn didn’t start The Diplomat planning to unveil Hal’s vice presidential career. But, the showrunner was exhilarated to watch Sewell explore his character’s unexpected political evolution. 

“It was really exciting seeing Hal step into this new role as vice president. The poor guy has been in Kate’s shadow for two full seasons and struggling, trying to be supportive,” Cahn says. “But we finally get to see him where he is most comfortable and has his feet on the ground. It’s exciting to see the dynamic that brought us to where the series begins, and Kate feels like Hal isn’t going to really be able to step back. Now we get to see why.”

So Kate isn’t Vice President? 

When The Diplomat Season 1 begins, it is clear the Wylers are only placed in London to give Kate a clear path to the vice presidency. In Season 2, she finally admits she even wants the job. But, it’s Hal who ultimately gets the gig in Season 3. Cahn suggests the disappointing development mirrors real world dynamics. 

“Kate is a woman who thinks she isn't ready for something even though she’s got the experience, competence, intelligence, and vision,” Cahn says. “But over time, she grows to understand that she can do this. She believes that she really can. And everybody who’s looking at her knows that she can. Still, it gets pulled away.” 

Yet, Kate makes the best of a bad situation. She gives Grace honest and useful advice as she becomes president. “Kate is at her best in a crisis. She’s about action. She’s about problem-solving,” Cahn continues. 

What exactly does President Penn have to do with the HMS Courageous explosion? 

Grace enters the presidency knowing she set in motion the devastating HMS Courageous attack, which killed 43 people on the warship. But, as she explains to Kate in the Season 2 finale, Grace only considered the attack to maintain the strength of the UK — and America’s nuclear safety along with it. Now, Grace must lead the United States, knowing what she did. 

“Grace feels really bad about the warship. But she made what she believed was going to be the lesser of two terrible choices. And it probably was,” Cahn says. “We’re not looking at people who have a good choice in front of them. They only have a series of terrible options in front of them. I wouldn’t want to be one of them. I am in awe of people who do have that kind of strength. It’s hard.”  

Is Roylin dead? 

Yes, Margaret Roylin (Celia Imrie) is dead — and her demise sends shockwaves throughout The Diplomat Season 3. Over Seasons 1 and 2, Kate comes to realize Roylin had a key role in the HMS Courageous explosion and its subsequent cover-up. “Roylin is a character who we introduce as though she's a villain,” Cahn admits. But, the longer Kate gets to know Roylin, the more complicated the political adviser becomes. 

“Kate comes to understand Roylin and begrudgingly respect her,” Cahn says. “Kate knows that Roylin made certain choices for her country. Roylin believes they were the right thing to do.” 

Yet, Roylin pays the ultimate price for her actions. By the Season 3 premiere, Prime Minister Trowbridge knows of Roylin’s connection to the Courageous attack. In response, the United States offers Roylin asylum in America. Rather than betray her country further, Roylin tragically dies by suicide toward the close of Episode 1.  

“Sometimes in order to be the person who rises to meet that moment, the game ends,” Cahn says. “Which is a terrible outcome. But Roylin considers herself a public servant and a servant to her country.” 

How did Hal and Kate get together? 

Cahn knew she wanted to do a flashback episode in Season 3. She got the perfect opportunity with Episode 2, which features Kate and Hal’s origin story. “There’s the fun of seeing what it was like when they first came together,” Cahn says. “There’s also a status [difference] that exists between Kate and Hal as long as we’ve known them in the series: She wants out of this thing, and he is committed to staying in the relationship. His love for her never wavers. He’s always working, consciously or subconsciously, overtly or covertly, to save that marriage.” 

Yet, their dynamic was once very different. As flashbacks reveal, before Hal and Kate were two of the most important politicos on the globe, they were colleagues in 2010 Baghdad, Iraq. Hal was Kate’s boss, and publicly very cool towards her to maintain the ruse. However, their covert operation was blown after Kate entered the office of her friend and colleague Carole Lengetti (Laurel Lefkow) with her underwear accidentally hanging out of the cuff of her pants. Carole — who also appears in Seasons 1 and 2 of The Diplomat — immediately put together the truth. 

So, the couple must make a decision — especially since Hal had just been reassigned to Vienna, Austria. Hal was ready to end the romance to protect and encourage Kate’s career. She, on the other hand, saw their potential to create the kind of relationship the Wylers end up having, where they regularly trade off whose career is the priority. 

“I really wanted to see the time when Kate was way more in love with Hal than he was with her,” Cahn says. “Through the flashbacks we see him get on board. He makes that move to, ‘Oh, this is a forever thing.’ Kate was there way before him.” 

By the end of the Episode 2 flashbacks, we watch Hal decide to propose. He took the twist tie off of a bag of party cups, and fashioned it into a ring for his bride. This is the same mysterious small red circle Kate is holding at the beginning of Episode 2, immediately after Hal is offered the vice president role. 

Why does Kate stay in London? 

Episode 2 ping pongs between Hal and Kate’s beginnings and their present-day decision-making around Hal’s vice presidential ascent. Kate is essentially told that to become second lady of the United States, she must resign as ambassador to the UK and pick up duties like local museum fundraising. While Stuart tries to push for a compromise — where Kate would keep her job and travel to D.C. whenever necessary — Hal says his wife would only do that if she wanted to be “3,000 miles away” from him. Suddenly, everyone understands the stakes of Kate remaining in London. 

Sadly for Hal, Kate admits she would prefer to be an ocean apart from her husband. She does not follow him onto the plane to America, and chooses her life in London over her marriage. While Hal might think Kate made a shocking decision, Cahn says the signs are there throughout Episode 2. 

“There’s something Kate knows in the pit of her stomach, and she’s fighting mightily against it,” Cahn says. “She wants to believe that she can be different than she is. That she can just roll with this and put aside what she's always wanted to do for somebody else’s path.”  

But, that’s not who Kate is. She’s a diplomat first, and someone’s wife second. “When Kate needs to walk toward the plane, it becomes clear to her that she’s done this too many times — told herself this story too often,” Cahn explains. “She wants to be her own person in the world. She does not want to be what she believes will be an ineffectual part of somebody else’s story.” 

Who is Callum Ellis? 

Callum Ellis is Kate’s new love interest, who enters her life in the wake of her separation from Hal. He is introduced in Episode 5 as a “bird watcher” or spy, who has pressing intel on Russian nuclear developments. In Episode 7 we learn Callum was in the armed forces before entering the intelligence field. Although Kate has previously had romantic and sexual tension with her colleague Austin Dennison (David Gyasi), Callum offers Kate her first real opportunity to explore those kinds of feelings outside of her marriage. 

“Callum is ready to meet Kate where she is. It's hot and sexy — fun and new,” Cahn says. “It surprises both of them that this relationship might have real legs.”

But, this pairing also has its challenges. As Kate slowly recognizes, Callum isn’t actually very different from Hal. Initially, the Diplomat team thought Kate’s new boyfriend needed to be the polar opposite of her husband. Then, they realized Kate wouldn’t stray so far from the familiar path.  

“I spent a lot of time thinking about what people do when they come out of a relationship with somebody that they desperately loved but wasn't good for them. They usually go and pick the same type of person.” Cahn explains. “Kate is not finding somebody else. She’s finding Hal without all the baggage that their relationship carries. She’s in that magical thinking period where you believe you can get the good things from a personality profile without the bad — like the rules of gravity won’t exist this time.” 

What is the Russian submarine? 

Upon meeting Callum, Hal finds out there’s more to Kate’s dashing new beau than meets the eye. Callum explains that Russia has lost contact with a submarine asset, and this isn’t just any watercraft. It’s a nuclear-powered B-90 Sarov sub that is “presumed distressed,” and possibly leaking radiation into British waters. 

Cahn decided to add the submarine threat to The Diplomat Season 3 after speaking with real-life civil servants. The writer came to realize that in a world where technology tracks our every movement and breath, submarines are “so damned difficult to actually find.” 

“We feel so surveilled constantly. So there’s this sense that everything in the world is knowable. Submarines aren’t. They’re hard to find. You can lose one and you're just fucked,” Cahn says. “The sub felt like a good way to use a single object to explore the powerlessness that a bunch of countries feel — and their inability to control a situation that is literally and figuratively explosive.”  

The danger of the Russian sub only increases for the remainder of Season 3. In Episode 7, Callum reveals the submarine is actually carrying the Poseidon torpedo, which is supposed to be a rumor (and years away from reality). But, in actuality, the Poseidon — “a salted bomb designed to maximize radio fallout” — is very real and lurking a mere 12 nautical miles off the northeast coast of England. In Episode 8, Grace and Kate both call it a “doomsday weapon.”  

What is “Runit Dome?” 

As Season 3 comes to its culmination, America and the UK must come to a consensus on how to deal with the nuclear submarine. Trowbridge is increasingly mistrusting of the United States. He seems poised to leave all negotiations — or accept help from China. Hal tells Kate to recommend a “Runnit Dome” scenario to appease Trowbridge, maintain American-British relations, and nullify the nuclear threat of the sub.  

As Kate explains, Runit Dome is located in the Marshall Islands. In the mid-1900s, Runit Island was used as an atmospheric nuclear weapons test site. In the late 1970s, Runit’s radioactive debris was encapsulated in concrete inside a crater on the island, and then covered by even more concrete to contain the dangerous materials. The Wylers are proposing Trowbridge do the same with the Russian sub — thereby taking it off the global chess board all together. 

Where do Kate and Hal stand? 

“In Season 3, Hal and Kate’s marriage is over as far as they’re both concerned. The two of them have gone through what we refer to as a divorce of the heart,” Cahn says. “Publicly, they’re still married. Privately, they know that they’re not, although Hal is never a give-up kind of guy.” 

These truths become clear after Kate stays in London at the end of Episode 2. She picks herself over her allegiance to Hal’s unique brand of chaos, which Kate thinks is “ruining her life,” according to Cahn. “It was important to see Kate go on the journey of, ‘I’ve loved this drug that is Hal. But it’s time to walk away and try and find my way in the world without that.’ ”

By Episode 5, Kate thinks she has found something new in the form of Callum. Yet, their relationship still falters. Kate realizes she may not treat people “the way they deserve to be treated,” and has more “baggage” than she assumed, as she says in the episode.

“For a lot of people, the consistency in your relationships is you. There can be a really horrifying realization when you see that the dynamics that you’re trying to escape have been created by you,” Cahn says. “Kate’s relationship with Callum is a way for her to have to confront herself in a more meaningful way when she no longer has the opportunity to blame Hal for the situation she’s in.” 

After getting a new perspective on her marriage, Kate apologizes to Hal and asks him to take her back. It’s the most emotional viewers have ever seen the diplomat. 

“It’s great and terrible to watch Kate understand what she’s done. It’s awful when it all kind of comes crashing down on her that she’s made such a big mistake, and she is just begging for another chance,” Cahn says. “That much clarity coming that fast is really brutalizing.” 

Mercifully, Hal immediately accepts Kate’s plea, promising her he’s “still here.” The couple share a passionate kiss and an intense hug. But, one small global issue has created yet another wrinkle in the Wylers’ marriage…

Who took the  Poseidon missile? 

For a few minutes in Episode 8, all is well in The Diplomat universe. Hal and Kate have seemingly saved Europe from nuclear disaster. Their marriage is on the mend. America and the UK are on the road to reconciliation. Then, Callum tells Kate that the Russians found their nuclear-powered sub … and the Poseidon is gone. 

Initially, Kate is worried that the Russians took the weapon back. Then, she pieces together the possibility that Hal and Grace have stolen the Poseidon — which would have disastrous effects on global politics. 

“That would make a lot of people really angry: both Russia, who made the weapon, and the UK,” Cahn says. “Hal and Grace are playing with matches in somebody else’s house. The repercussions are not going to only land on America — they’re going to land on another country. It’s really going to make a lot of people unhappy.” 

As the Season 3 finale comes to a close, Kate reckons with the evidence of Hal and Grace’s deception. She realizes that Grace told Trowbridge that the US sent an unmanned drone to take photos of the sub. But, she actually sent an Ohio-level submarine big enough to hold the Poseidon. And then there’s the matter of Hal and Grace’s conspiratorial glances and late-night meetings. When Kate points these details out to Hal, he orders her to “tell no one.” Hal then immediately whispers to Grace that Kate “knows” what’s afoot.  

So, just how “productive” are Hal and Grace? 

Kate’s confidence in her marriage is torpedoed by Todd Penn, the president’s husband. As Kate and Todd watch Hal and Grace take photos together, Todd points out their spouses’ “extremely productive working relationship.” While Todd and Kate accept Hal and Grace’s connection isn’t sexual, Cahn says viewers should still be alarmed by this duo’s professional potency. 

“Hal and Grace are two really powerful, dynamic people who are feeding off of each other’s intellect and intensity,” Cahn says. “There’s a lot to worry about there.” 

In fact, Kate may just be more concerned about her husband teaming up with Grace in the office over the bedroom. “It’s a much, much, much bigger betrayal,” the writer says. “We see it in Todd — how devastating it is to witness that spark in your loved one’s eyes when they’re literally changing the world with someone else. It’s intoxicating.” 

This situation is particularly dangerous since Grace is not the dampening force for impulsive Hal that Kate is. Grace is the kind of person whose actions lead to the HMS Courageous explosion. “When two people are pushing the gas at the same time and nobody’s got their foot on the brake — it’s scary,” Cahn says. 

Find out what new political machinations Hal and Grace will create together in The Diplomat Season 4, which has already been announced. Until the Wylers and Penns return, keep coming back to Tudum for all your Diplomat news. 

Watch Keri Russell and Rufus Sewell Discuss The Diplomat Season 3