The Diplomat Season 3 Ending Explained: Breaking Down That Wild and Twisty Finale

The Diplomat has returned to Netflix for its third season, and the blockbuster series is still as twisty and impossibly compelling as it ever was. After spending the first two seasons unraveling a conspiracy and finally unmasking its architect, Kate and Hal Wyler (Keri Russell and Rufus Sewell) suddenly find themselves stuck in the middle of something they were extremely unprepared for. 

Like the first two seasons, Season 3 of The Diplomat has a doozy of an ending, one that had me shrieking at my screen in simultaneous awe and horror. It can be a lot to take in, so let's go ahead and break it down.

Warning: This article contains many spoilers for Season 3 of The Diplomat, including a detailed description of how it ends.

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What happened at the end of Season 3 of The Diplomat?

At the end of Season 2, Hal Wyler told President Rayburn (Michael McKean) that Vice President Grace Penn (Allison Janney) came up with the idea for the false flag attack against a British aircraft carrier — and then Rayburn responded to the revelation by immediately having a heart attack and dying. That made Grace Penn, who was positioned as the main villain of the series to that point, the new president of the United States.

As Season 3 begins, Hal, being the pure political animal that he is, immediately pivots to obsequiosness — Grace Penn is now his boss, so it no longer matters that she's the bad guy. Hal makes a play to have Kate be Penn's new VP, but Penn picks Hal himself instead. Kate feels burned by that choice, but it's not hard to figure why Penn made it. Hal is an attack dog who will do anything for the sake of a political win, whereas Kate is an idealist who cares about right and wrong. 

While their marriage has been rocky since before the series began, Hal being picked for the VP slot appears to truly be the last straw — Kate returns to London to continue being ambassador, while Hal stays in D.C., and from then on, as Hal makes very clear to Billie Appiah, he and Kate have a marriage in name only. The difference between this and their past estrangements is that it also breaks up their political partnership — Grace Penn is Hal's new political wife, so to speak.

President Penn, meanwhile, seems hellbent on clearing up the whole mess about the false flag attack on the carrier, because Margaret Roylin, the one who organized and carried out the attack, decided to kill herself while in CIA custody. But instead of admitting her part in things, she blames the deceased President Rayburn for it — this was Kate's idea. But that move backfires when British prime minister Nicol Trowbridge (Rory Kinnear), who had been mentored by Roylin, tells everybody that it was Rayburn's plan and leaves Roylin out of the story entirely.

And then, after the fourth episode, we get a five-month time skip. 

The back half of the season, then, examines the fallout of both the change in the Wylers' relationship, and the change in the relationship between the United Kingdom. and the United States. Kate is now involved with a hot British spy named Callum (Aidan Turner) who has learned that Russia apparently wrecked a nuclear submarine in the North Sea near the English coast. To make matters worse, the sub contains a secret Russian superweapon called Poseidon that could permanently irradiate a large chuck of the ocean.

Our heroes, and also Grace Penn, try to warn Trowbridge about the situation, but he doesn't believe them. Kate has several other ideas for how to convince the PM, including burning her spy boyfriend and illegally sending in an American sub to take pictures — Hal and Penn end up endorsing the second plan, but with Hal injecting some of his own ideas that aren't revealed until the final scene of the season. We'll circle back to that in just a moment. 

Even after they show Trowbridge the pictures, he doesn't have any interest in accepting American help in dealing with it. So Kate suggests something else: burying the sub on the bottom of the ocean with concrete that would contain much of the vehicle's radiation. And that seems to be something Trowbridge can agree with. 

But there's just one problem. Callum tells Kate that radiation levels have dropped very rapidly in that part of the ocean, which almost certainly means the nuclear superweapon in the sub has been moved. Callum thinks it was the Russians recovering their lost equipment, but Kate immediately realizes the truth: Hal did this. The tell had actually come earlier, when they presented the images of the sub to Trowbridge — Penn told him the pics were taken by a drone, rather than the submarine Kate had suggested sending. Penn lied to Trowbridge about the vehicle in order to mask the fact that they'd sent in a sub capable of taking the Poseidon weapon.

To sum up: A Russian nuclear sub with an atomic superweapon aboard crashed at the bottom of the ocean off the coast of Great Britain, and the US government stole it at Hal Wyler's behest. Yikes!

What does The Diplomat Season 3's ending mean?

There are a number of pretty major potential consequences to this move, but it's also genuinely possible, as Hal says to Kate in the final scene, that no one will find out that they did it. That's the real purpose of the "bury it in concrete" suggestion from Kate, who was manipulated by Hal into making it. The idea would be that the sub ends up buried before anyone can look closely at it, and then everybody assumes that's the end of the story without actually confirming.

There's just one pretty big problem. Callum, and the folks he's been coordinating his nuclear radiation data with, are pretty sure that somebody took the Poseidon weapon, but they obviously don't know it was the U.S. Since Hal's plan relies on nobody knowing that anything happened at all, at least in the short term, that makes Callum a huge wild card. As Kate notes in the finale, if China, Russia, or the U.K. discover that anyone did anything with the Poseidon weapon, it could spark a war. It's a pretty scary situation.

Likewise, it's a pretty dramatic shakeup to The Diplomat's relationship dynamics. Kate and Hal are now fully opposed to each other for the first time since the series began, with Hal manipulating Kate the same way he manipulates everyone else. And now Kate has to navigate this incredibly fraught situation with her new boyfriend, who's found the smoking gun. 

This is the natural endgame for Hal, a completely amoral person who'd only ever done anything good because of Kate's influence. Without her, he reverts to being a feral political animal.

In the midst of all this, the finale has a scene with Eidra (Ali Ahn), the local CIA station chief, and Kate's chief of staff Stuart (Ato Essandoh), where they discuss Stuart's unwitting participation in the Wylers' attempts to depose Grace Penn back during Season 2. Stuart is considering getting a lawyer in case there's any blowback about all that, and Eidra explains that if he tries to be a whistleblower or preemptively protect himself, the government will simply destroy his entire life — she's saying this not as a threat, but because she cares about him and doesn't want him to be consumed by the machinery of the U.S. government. 

That scene exists for a very important reason: it's laying out why all the players in this story are trapped in it and will have to ride it out to the end, for better and worse.

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What's the play here for Hal and Grace?

The move to steal the Poseidon weapon is a response to Trowbridge's constant combativeness since Roylin's death. Trowbridge plans to hold Rayburn's alleged involvement in the carrier attack over the U.S.'s head forever — during Season 3, the U.S. and U.K. have the same sort of breakup that Hal and Kate did, with the U.K. having the public upper hand in the feud after Rayburn took all the blame for the attack. 

But with the Poseidon weapon now in U.S. hands, Penn and Hall have leverage. If China or Russia discover that the weapon was stolen, they'd naturally assume the Brits took it since it was in their waters. The U.K. would be in dire need of American support if that were to happen, and it would allow Penn to potentially reverse the damage done by Rayburn's reported involvement in the carrier attack. But there's still one more big variable in this house of cards: the fact that it was actually Grace Penn who had the idea for the carrier attack. They'll still have to keep that secret, too.

What's the play for Kate?

Kate is in a situation she's not used to at all. Not only does she not have Hal's help in dealing with stuff, he's actually the enemy now. To make matters worse, opposing him could damage American interests, which is the opposite of what Kate wants. For an idealist, it's an impossible situation.

But she's not without allies. Kate's biggest strength is that she doesn't play like a politician — she just cares about getting stuff done and taking care of her people. That earnestness is what draws people to her, and it's why she has more friends in low places than she probably realizes. If she's going to successfully navigate this horrible mess, she'll need folks like Eidra and Stuart on her side in this new conflict between the employees and their bosses. 

She probably also has an ally in Todd Penn (Bradley Whitford), the First Gentleman, who's been raising a stink all season both about Hal and Grace's new relationship, and how he gave up his own political career to serve in the Rayburn administration as Grace's husband. There's some pretty clear bitterness there, and it's not very likely he'd be on board with his wife's shenanigans.

Kate's relationship with Callum will be another sticking point, since he's been a crucial player in this situation. Will she tell him everything? Or will she have to keep secrets? It's hard to figure how Kate will handle this, but it won't be the way Hal would, that's for sure.

All three seasons of The Diplomat are now streaming on Netflix. The series has been renewed for a fourth season.