This post contains spoilers from The Diplomat season 3 episode 8 from this point forward.
After a tense season of The Diplomat, it all comes down to the season finale. The penultimate episode found Kate navigating the tricky waters of Trowbridge publicly blaming Rayburn for the carrier attack and learning from Callum that China could be close to capturing the submarine containing a weapon known as Poseidon. She's able to save her relationship with Callum, but what's next?
The Diplomat season 3 episode 8 begins with Penn returning to London and greeting Trowbridge for the upcoming summit. Hal secured a separate room at the manor for Kate so she can be with Callum, and they have yet another fight. Meanwhile, Todd apparently believes that Penn has some romantic feelings for Hal, which she doesn't appreciate.
Penn and Trowbridge begin their private meeting with differing opinions about the Rayburn situation, but Penn broaches the subject of the submarine. Trowbridge believes Poseidon to be a fictional story, but Penn insists it's real and advocates against allowing China to retrieve the sub. Trowbridge leaves the room without a word. Kate's photo with the spouses gets interrupted by this development.
Kate reacts to Dennison's surprise marriage
Trowbridge thinks that Penn is lying about Poseidon, which prevents him from wanting to work with the administration. Kate turns to Callum to help convince Trowbridge, but he's not inclined to help since it could result in him getting in deep trouble and the execution of his source. Callum wonders if this was Hal's idea in order to get him away from Kate. Once again, they're on bad terms.
Before the event, Dennison introduces Kate and Hal to his wife Thema (Tracy Ifeachor), and it's not as awkward as you would have expected. Meanwhile, you could cut the tension at the dinner table with a knife. Kate steps out to talk with Callum, who agrees to tell Trowbridge that the Russians came to him about Poseidon. She promises to protect him and his source.
When she relays the message to Hal, he turns down using Callum as an option. Hal thinks that Kate was "moved" by Callum's "noble sacrifice." He doesn't want to ruin her happiness with him, and he's surprised that using Callum was her idea. Kate and Hal next present Penn with a new option: Sending an American submarine down to take pictures. She's not thrilled at the thought of crossing Trowbridge.
Who stole the Poseidon from the submarine?
Kate and Dennison clear the air about the personal tension between them, and he explains how he came to marry Thema and opens up about his first wife dying by suicide. He's one of the few that doesn't know she and Hal are separated. Speaking of Hal, he arrives with the news that Penn agreed to sending down an American submarine. Elsewhere, Eidra talks Stuart down from overreacting.
Todd continues to be insecure about his wife's close working relationship with Hal, which creates unnecessary conflict between them. Kate gives Callum the relief that he doesn't have to throw away his career, and their problems are all but resolved. She asks him to leave her room under the guise that she needs to return to work, but she does not.
Penn delivers the photos of the submarine to Trowbridge hoping to build trust between them. Unfortunately, he doesn't agree to their offer to take care of it. An argument ensues, and Hal instructs Kate to be the voice of reason. She whispers in his ear to bury the submarine and weapon in concrete on the ocean floor like Runit Dome, insisting that no country she be in possession. He agrees.
Kate asks Billie if she can join them on the return trip on Air Force Two, and when Billie and the team leave the room, Kate begins crying and apologizes to Hal. She asks him to take her back. They kiss and he promises her that he's still here for her. Before they leave, Callum informs Kate that the Russians stole the Poseidon. Kate realizes that Penn and Hal could be the ones who stole the Poseidon from the submarine. He confirms it's true as she worries a war could erupt.