Kate’s Affair With Callum Ellis Torpedoes Her Alliance in The Diplomat Season 3 Episode 5

Episode 5 of The Diplomat Season 3 throws a match into Kate Wyler's carefully managed life. A surprise anniversary visit from her husband, newly appointed Vice President Hal Wyler, lands right in the middle of a swanky Winfield House party and collides with Kate's very complicated situation with Callum Ellis. It is messy, it is tense, and it is absolutely the show operating in its sweet spot: when personal choices start warping national security calculus.

Episode 5: Party crashers and fallout

Hal flies to London to surprise Kate, and instead finds out she's involved with Callum Ellis, a spy carrying urgent intel on Russian nuclear activity. Their marriage has been more strategic than romantic for a while, but this revelation still hits a nerve and exposes just how brittle their alliance really is.

The problem isn't just the optics. Kate's entanglement with Callum is tangled up with sensitive information about a missing Russian submarine and something referred to as the Poseidon weapon. That crossover isn't hypothetical; it could put Callum in real danger. The episode keeps pushing the same point: Kate's private life and public job are not separable, no matter how much she tries to compartmentalize.

So... who is Callum Ellis?

Introduced in Season 3, Episode 5 (Birdwatchers), Callum Ellis is played by Aidan Turner and shows up as Kate's new romantic interest after she and Hal quietly split in private. He is not just a rebound with great hair. He tells Hal he's been coordinating with North Sea nations to locate a missing Russian B-90 Sarov submarine, which may be leaking radiation into coastal waters and affecting Britain and other countries. That is a lot of responsibility for someone who also just walked into the middle of a vice presidential marriage crisis.

• First appearance: Season 3, Episode 5 ('Birdwatchers')
• Played by: Aidan Turner
• Background: Former armed forces, now in intelligence
• Current mission: Working with North Sea nations to track a missing Russian B-90 Sarov sub possibly leaking radiation into nearby coasts
• How deep he is in: He volunteers to personally brief the UK Prime Minister on critical submarine intel in the finale, which is a bold (read: risky) move
• Where he stands with Kate (Keri Russell) by season's end: Still her confidant, bringing her updates like the discovery of the missing sub and the disappearance of the Poseidon weapon
• The catch: The finale leaves things murky, which is not great for a relationship built on secrets and proximity to power

Why he feels familiar: the creator's take

Creator Debora Cahn has been pretty candid about what Callum represents for Kate. The short version: he's Hal, minus the miles of baggage.

"I spent a lot of time thinking about what people do when they come out of a relationship with somebody that they desperately loved but wasn't good for them. They usually go and pick the same type of person."

"Kate is not finding somebody else. She's finding Hal without all the baggage that their relationship carries. She's in that magical thinking period where you believe you can get the good things from a personality profile without the bad, like the rules of gravity won't exist this time."

That lines up with what we see across the back half of the season. In Episode 7, an argument with Callum forces Kate to stare down her own baggage. By Episode 8, Dennison helps her reframe what her marriage to Hal actually is, and she does the thing that surprises even her: she apologizes and asks Hal to take her back.

Where Season 3 leaves everyone (and what it sets up)

By the finale, Callum is still feeding Kate crucial intel — including that the missing submarine has been found and that the Poseidon weapon has vanished — but he does not know the full extent of Kate's personal and professional shifts. Combine that with the finale's shock ending, and their dynamic is anything but settled. Translation: expect tension to spill over into Season 4.

The Diplomat Season 3 is streaming on Netflix. Where do you think this leaves Kate and Callum — and how much trouble does that Poseidon situation spell for both of them?