In the explosive third season of Netflix’s The Diplomat, a single word drives the action and threatens to start a war: Poseidon. This mysterious “doomsday-level” weapon, hidden on a sunken Russian submarine off the coast of England, becomes the center of a dangerous power struggle between the United States and the United Kingdom. The hunt for Poseidon tests alliances, destroys trust, and leads to a stunning betrayal that changes everything for Ambassador Kate Wyler. Just what is this weapon, and why does it cause so much trouble? The answers reveal a plot twist that connects personal drama to global disaster.
What Exactly Is the Poseidon?
In the world of The Diplomat, Poseidon is not a myth—it’s a terrifyingly real threat. The weapon is described as an underwater drone, powered by its own small nuclear reactor and armed with a nuclear warhead. Its design is not for a pinpoint attack. Instead, it is meant to create a massive, radioactive tsunami capable of contaminating huge coastal areas with fallout, making it a weapon of unparalleled destruction.
The crisis begins when MI6 agent Callum Ellis informs Kate Wyler that a Russian submarine carrying Poseidon has sunk in British waters. With the crew dead, the nuclear weapon is just sitting on the ocean floor, a prize waiting for the first nation to grab it. For President Grace Penn and Vice President Hal Wyler, the immediate fear is that either Russia will recover it or, worse, that British Prime Minister Nicol Trowbridge will cozy up to China and give them access.
Prime Minister Trowbridge, however, is deeply skeptical. His trust in the American government was shattered after the cover-up of the HMS Courageous bombing. When Grace Penn warns him about Poseidon, he dismisses it as a “fictional tool,” believing it might be a trick to make the UK submit to American demands.
Grace Penn explains the Poseidon to Trowbridge as: “It’s an underwater drone powered by a small nuclear reactor armed with a nuclear warhead and designed to contaminate an unusually large area with radioactive fallout”.
To prove the weapon’s existence, Kate devises a plan to send an American submarine to photograph the wreckage. The photos are presented to Trowbridge, who remains suspicious but is eventually convinced to work with the Americans on a neutral solution: burying Poseidon under concrete on the ocean floor, where it can rot away safely.
The Stunning Twist: Who Really Has the Poseidon?
Just when a peaceful solution seems within reach, the season delivers its masterstroke of betrayal. Callum Ellis returns to Kate with alarming news: radiation readings at the submarine site have dropped suddenly. Someone has already taken the Poseidon weapon.
At first, Kate suspects the Russians. But a chilling realization dawns on her as she watches her husband, Hal, share a conspiratorial look with President Penn. She pieces together that the “unmanned drone” the US sent to take pictures was, in fact, a much larger Ohio-class submarine—one big enough to secretly retrieve the massive weapon. Hal and Grace had used Kate’s own plan and her credibility with the British government to steal Poseidon right from under their ally’s nose.
In the finale’s closing moments, Kate confronts Hal. He does not deny her accusation. His only response is a warning to President Penn: “She knows”. The season ends with Kate standing beside Todd Penn, Grace’s husband, as they watch Hal and Grace pose for photos—a united front hiding a dangerous secret.
This theft is not just a political maneuver; it’s an act that could be interpreted as an act of war by the British government. For Kate, the personal betrayal is even deeper. After a season of struggling with their separation and starting to reconcile, she discovers Hal used her to execute a deceptive and reckless plan.
Actress Keri Russell, who plays Kate, described the moment as “chilling,” noting, “He basically uses her to get through some information to the prime minister and then he’s hatched some shady plan with the president. That is not what they discussed”.
How Poseidon Mirrors Real-World Fears
In a strange and unsettling coincidence, the fictional Poseidon has a direct counterpart in the real world. In late October 2025, as The Diplomat‘s third season was streaming, Russian President Vladimir Putin gave a speech boasting of new military technology—an underwater, nuclear-powered drone designed to deliver a warhead that could create a radioactive tsunami.
The real Russian weapon’s name? Poseidon.
This parallel made the show’s storyline feel less like fiction and more like a tense prediction. While the show’s creators did not have secret intelligence, the coincidence highlighted how the series taps into genuine global anxieties about new, uncontrollable weapons and the fragile nature of international trust.
What Poseidon Means for Kate, Hal, and Season 4
The stolen Poseidon weapon completely rewrites the rules for all the main characters:
- For Kate Wyler: She is now complicit in a secret she finds morally reprehensible. Her career as an honest broker is in jeopardy, and her marriage is based on a fresh betrayal. Actress Keri Russell summed up Kate’s dilemma: “I’m a part of it! I’m complicit in it, because I’m lured back in”.
- For Hal and Grace: They have formed a powerful and dangerous alliance. Allison Janney, who plays President Penn, suggests Grace chose Hal as VP because he operates in the “gray” areas of politics, much like she does. They believe they are the right people to control such a weapon, but their actions have isolated the UK and possibly their own staff.
- For the US-UK “Special Relationship”: The alliance is in tatters. Creator Debora Cahn described Season 3 as “the split,” both for Kate and Hal’s marriage and for the two countries. If Prime Minister Trowbridge discovers the theft, the diplomatic fallout would be catastrophic.
The stage is set for a tense fourth season, which Netflix has already confirmed is in the works. The central question will be: What will Kate do with her devastating knowledge? Will she expose Hal and Grace, potentially causing an international scandal and war, or will she be forced to live with their secret?
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