It's been one year since The Diplomat season 3 finale dropped a bomb (pretty much literally) on Kate Wyler. Since then, we have been waiting to see what happens next in season 4 for the Penn-Wyler administration after Kate learns their secret about stealing the Poseidon missile. How will Kate navigate the minefield surrounding her husband and the President of the United States?
The Diplomat season 4 episode 1 picks up almost immediately after the season 3 finale as the administration rides in a motorcade. There's obvious tension in the silence between Kate and Hal before she breaks it to ask how the order went out about stealing the weapon. He initially shoots down her questions before he answers her follow-up about Billie being involved in the order.
There's a demonstration outside the airport and blocking the highway that causes their motorcade to be rerouted to Windsor Castle for safety. Once they arrive at the castle, Kate pulls Billie aside to inform her that Callum Ellis thinks the Russians took the weapon due to a drop in radiation levels near the submarine. Billie assures Kate that she will look into it and prepare Kate a response.

Kate pitches the president to reverse course on Poseidon
After being welcomed and greeted with a salute, the administration is shown to their temporary offices inside the castle. Hal confronts Kate about talking to Billie since Grace already thinks she's a security risk. But Kate was warning Billie that Callum knows the weapon is missing and they need to explain why before he tells Trowbridge it was the Russians.
Kate's worried that the Russians will attack the United Kingdom and questions the decision-making between Graceand Hal regarding their alliance with Trowbridge. They continue to fight over the what ifs and intricacies of the decision. Hal accuses Kate of being mad she was out of the loop and throws their break in her face. But she continues to express her concern that Hal and Grace are dangerous.
In the middle of their argument, Kate gets a splinter and they're suddenly like any married couple solving a problem together. Following up on Hal's accusation that she's jealous, she clues him in on Todd being jealous. Kate offers to have Callum come to Windsor for a conversation, perhaps the truth, and asks Hal to get her in the room with Grace to propose the meeting.
Before she gets the go-ahead, she calls Callum about dropping by Windsor. Back at Winfield, Stuart unlocks the study for Eidra to have a meeting with Tom Libby. Kate takes her own meeting with Grace and quickly delivers her idea to tell Callum the truth about Poseidon. Grace shoots it down, but Kate persists that Callum will know Kate's lying. Shot down again, Kate scoots her chair closer to Penn.
She tries a new strategy that pokes holes in Hal's charm and alludes to the closeness of Hal and Grace. Kate reveals that Todd notices the closeness as a last ditch effort. Stuart spreads the word about "Amazing Grace" to their guests while Eidra and Libby privately talk about burying the sub in concrete in the ocean. Grace approaches her husband about Kate's words with frustration.
Grace and Hal talk alone for the first time since Grace was presented with their spouses' concerns. When the subject comes up, Hal apologizes for Kate bringing it up, but she corrects him that Todd actually brought it up with Kate. However, the tension subsides when an innuendo sends them both in to a fit of laughter, and it's clear the concern isn't totally unfounded.

An assassination attempt leaves Grace's life on the line
Nevertheless, Hal isn't happy with Kate, who's having trouble making both Grace and Hal understand the point she was trying to make. In their defense, she's not being completely clear with her words about what she was trying to explain. Hal attempts to give her a way out of their marriage given how she has been making him feel. But that's not what she wants.
He doesn't believe that she's trying to get back into their marriage. It's clear that she wants to be his confidante and to be his partner. She admits that Callum helped her realize that she "hits too hard" with her words and that's something she wants to work on. Hal still feels as though he can't live in fear that Kate's constantly trying to end their marriage every time they disagree.
Billie interrupts them to instruct Kate to let Callum know Intelligence deems the radiation levels as stable and his reading must be faulty. Grace officially shot down Kate's idea of reading Callum in, which is unfortunate considering he's en route to Windsor. Kate seems intent on getting in Grace's ear one more time, which she manages to do in the castle's bathroom.
She starts with an apology before launching into her pitch about making a mistake not using Callum as a resource. When she explains that she doesn't want to lie to Callum, Grace resolves that she will find someone who can. They run into Hal, and Kate's unfortunate joke (maybe?) about Grace wanting to fire her gives the president an idea since their arrangement isn't particularly flawless.
Hal gives Grace permission to fire Kate as ambassador, which strikes her in a way that's unreadable. Eidra and Libby continue their discussion about finding a reason why they're burying something in concrete. Hal assures Kate that she won't be fired. His words seemingly worked as reverse psychology on Grace. Kate meets with Callum, tells him the lie, and distracts him with a breakup.
After her chat with Callum, Kate takes Hal to the church and essentially renews her vows. The administration finally makes their way out of Windsor, but while walking across the lawn and saying their goodbyes, Grace gets shot right before getting into the car. Kate rushes into action and pushes Hal down as the guards pull Grace to the ground to avoid another shot.
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