Keri Russell returns as US ambassador Kate Wyler in the third season of The Diplomat, only this time with some new responsibilities.
Created by Deborah Cahn, the political thriller picks up in the aftermath of season two, as Grace Penn takes over as president. Meanwhile, Kate’s husband Hal is now vice president, leaving Kate to scramble between life as an ambassador and the second lady.
Rufus Sewell, Allison Janney, David Gyasi, Ali Ahn, Ato Essandoh and Rory Kinnear all return for season three. Newcomers to the cast include Bradley Whitfield and Aidan Turner.
All eight episodes of season three were released on Netflix on October 16, 2025.
Who ends up with Poseidon in The Diplomat season 3 finale?
In the final episode, US president Grace Penn attempts to convince UK prime minister Nicol Trowbridge that the Russian submarine submerged 12 miles off the UK’s coast actually houses a nuclear weapon called Poseidon. Nicol is coordinating with China to help remove the submarine, but as Grace explains, this is effectively handing them a nuclear weapon. Instead, she wants the UK prime minister to cooperate with the US in its retrieval.
Nicol leaves the meeting unconvinced that Poseidon actually exists. However, Kate Wyler convinces her British spy lover Callum, who knows the Poseidon is real, to tell Nicol about his sources, a move which would cost him his career. Kate’s husband, vice president Hal, however, refuses to let Kate and Callum go ahead with this proposal – which leads them to form an alternative, far riskier, plan with the president.
To convince Nicol, Kate suggests to Grace that they should send a US submarine to take pictures of the Russian submarine to prove Poseidon is real. As Grace explains, without permission from the prime minister, entering British territorial waters could be seen as an act of war, so it’s a risky proposition. After Kate makes her pitch, she leaves Hal and Grace to ruminate on the idea, before they eventually give it the green light.
Sometime later, Grace presents the photos to Nicol, and while he’s more convinced of Poseidon’s existence, he doesn’t trust the US after they went behind his back to retrieve the information. Nicol refuses to cooperate with the US, but under Hal’s encouragement, Kate intervenes and tells Nicol that no nation – whether China, the UK, US, or Russia – should have Poseidon, so he should bury it under the sea. Nicol, trusting Kate, agrees.
While it appears as if everything has been settled, Callum informs Kate that Russia has seemingly already stolen the Poseidon. Kate quickly informs her husband, but a chat with Grace’s husband Todd, about the close relationship between Hal and Grace of late that he’s become envious of, makes Kate realise what’s actually happening. Russia hasn’t stolen the weapon at all, it was actually the US, in a secret plan concocted by Hal and Grace behind closed doors.
Kate confronts Hal about her suspicions. “You used me to sell a lie to the prime minister,” she remarks. “When the Brits find out, they will consider this an act of war. So will the Russians.”
After Hal demands Kate stay quiet about the plot, he joins Grace for a photo opportunity on the steps. In the final scene, we see Hal remark to Grace about a problem; specifically that Kate knows about their secret. The episode ends on an unsettling note with a potential war on the horizon, and with Kate not knowing who she can trust.
Will The Diplomat return for a season 4?
The Netflix show was renewed for a fourth season back in May. No release date has been announced, but based on the rollout of prior seasons, it will likely arrive in 2026.