'The Diplomat' Season 3 Ending, Explained: Breaking Down the Shocking Twists

For three seasons, the Keri Russell-starring political thriller The Diplomat has contended with a universe in which the Americans have double-crossed their closest allies, the U.K., through the actions of Vice President Grace Penn (Allison Janney). In season 3, the fallout from that betrayal becomes more threatening after President Rayburn (Michael McKean) dies suddenly, and Penn is promoted to the highest office. Despite some officials knowing the truth, including Russell’s Kate Wyler and her husband Hal (Rufus Sewell)—both career diplomats—their quests for political gain are also part of the equation.

The Netflix hit’s latest season, which premiered on October 16, 2025, tests the Wyler marriage when Hal is appointed to the Vice Presidency over Kate, and again when Kate discovers his collusion with Penn during a heated international negotiation over a deadly weapon called the Poseidon in the finale. Does their marriage survive the acts of betrayal, and can the alliance between the U.S. and the U.K. ever recover? Read on to find out how season 3 of The Diplomat ends, and how it sets up its already confirmed fourth season.

Is President Penn able to reinstate the alliance between the United States and the U.K. at the end of 'The Diplomat' season 3?

While the U.K. Prime Minister Trowbridge (Rory Kinnear) might believe that he landed on neutral ground with the Americans, the final moments of The Diplomat season 3 suggest something different. Looming over the two governments are the claims pinning the British warship attack on the late President Rayburn (which are false, as President Penn was the true mastermind) and Kate’s intelligence around a Russian submarine carrying a large-scale weapon called the Poseidon. If it detonates, it threatens to obliterate a large portion of the country. The only problem is that the trust between the two countries is so weak that the Prime Minister doesn’t believe the President. As the link between the two countries, Kate is determined to find a solution.

Initially, she convinces her fellow Ambassador and new lover, Callum Ellis (Aidan Turner), to take the fall, as he’s in good graces with the Prime Minister and holds the key to the intelligence. He initially refuses to give up his career and his source, but eventually agrees. However, Hal vehemently opposes the idea, forcing Kate to suggest something different. Using American subs stationed nearby, the government will send a drone to take photos of the submarine to prove its existence.

In the board room, things get heated when the Prime Minister doesn’t believe the photos are real and questions why he should accept help from the U.S., which has already crossed them once. Hal encourages Kate to use her sway over Prime Minister Trowbridge to convince him to “Runit Dome,” referencing a previous mission in which they buried a similarly threatening weapon in concrete. He agrees and shakes hands with President Penn. But the high doesn’t last long—Callum’s intelligence about the dropped radiation levels tips Kate off to a plan that Hal and Grace concocted without her. They took the “genocidal weapon” for themselves, setting up an even larger-scale conflict for season 4 of the Netflix original.

Does Kate and Hal’s marriage survive 'The Diplomat' season 3?

Much of the third season of the political thriller contends with former VP and now-President Grace Penn’s decision to promote Hal Wyler to the Vice Presidency rather than his wife Kate. The fallout from this rocks their marriage, which they decide to dissolve behind closed doors. Kate agrees to be the Second Lady publicly while they live separate lives in private, with Hal in Washington and Kate in the U.K.

During this private separation, Kate begins seeing other colleagues. She and Foreign Secretary Dennison (David Gyasi) attempt to hook up before he pulls away citing bad timing, and months later Kate has moved on to a fellow Ambassador, Callum Ellis. Hal is acutely aware of Kate’s extracurricular relationships and makes a few digs at them even though he maintains that he just wants her to be happy. When Kate finally takes him at his word, she realizes she wants to make things work, and she makes a gesture to save their union: She asks Hal’s chief of staff, Billie (Nana Mensah), to find a seat for her on Air Force Two back to Washington and apologizes to Hal.

Their seemingly united front comes in handy while negotiating with the Prime Minister regarding the Poseidon. However, it’s almost immediately called into question again when Kate realizes that Hal and Grace instructed the Navy to take the weapon for American use.

Do other White House staff know what’s going on in 'The Diplomat' season 3?

Most of what happens in Kate’s world is on a need-to-know basis, which means many of her staffers and close confidants are often left in the dark. When the plot to blame the British aircraft attack on the late President Rayburn instead of the British campaign manager Margaret Roylin (Celia Imrie) comes to light, Stuart (Ato Essandoh), the deputy chief of mission of the U.S. embassy in London, has a crisis of conscience. He can’t fathom working for a government whose values don’t align with his own. CIA Station Chief Eidra Park (Ali Ahn) convinces him that if he leaves on those terms, he’ll be unemployable elsewhere. But her motives aren’t entirely about his job prospects—when she finally tells him she wants him to stay, she climbs into his lap and makes out with him.

Does Grace Penn's husband Todd trust Hal in 'The Diplomat' season 3?

Bradley Whitford joins the cast opposite his West Wing colleague Allison Janney as Todd, the husband of the President. He appears sparingly until the end of the season (and will be a series regular in season 4), when he begins showing signs of jealousy towards Hal and his closeness to his wife, mostly spurred on by his simultaneously “alive and dead” career as a biologist. Kate is convinced that Hal and Grace have a good working relationship, but after she uncovers their plot to steal the Poseidon, she finds herself listening to Todd’s words a bit more closely. “Nothing to worry about, right?” he says rhetorically. And while they probably aren’t having an affair, it’s clear that their partnership can’t be trusted.