Netflix’s The Diplomat has built its reputation on high-stakes geopolitics and razor-sharp dialogue. One mysterious term from the recently released third season has begun to dominate fan discussions and political intrigue: Poseidon. Poseidon is introduced as a shadowy element in the unfolding narrative that could redefine the power dynamics at the heart of the show.The Diplomat Season 3 seems determined to explore the consequences of secrets that are too big to stay buried. The emergence of Poseidon hints at choices that could destabilise governments and put lives at risk. The name itself (drawn from the Greek god of the sea) suggests unpredictability and immense power.In The Diplomat, Poseidon is a nuclear weapon located on a Russian submarine off the coast of England. The weapon is described as 'doomsday-level', capable of unleashing war between nations.More on this in our story.What is Poseidon in The Diplomat? View this post on Instagram Instagram PostPoseidon became an important weapon and element in The Diplomat Season 3, after Kate, Hal and Grace discovered its whereabouts off the coast of England. What exactly is Poseidon in Netflix's popular political thriller series? In a meeting with the UK Prime Minister, Nicol Trowbridge, Grace explains the Poseidon 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.''Both Kate and Grace call Poseidon a 'doomsday-level weapon', capable of bringing destruction and war between nations. While Trowbridge and the UK were suspicious of Poseidon's existence in The Diplomat Season 3, Kate provides them with photographic evidence, proving its existence and imminent threat. However, since this Russian-made weapon was tucked away off a coast in England, Grace and Hal wanted Trowbridge to hand it over to the Americans, as they have the necessary technology to destroy it.Since Trowbridge had lost trust in his American counterparts after the harrowing scandal in the second season, he did not want to hand them another nuclear weapon. This is when Hal asked Kate to convince Trowbridge to let the US send an unmanned drone into the ocean to collect pictures of the Russian submarine. After this photogenic evidence, it was decided that the US would help the UK bury the Poseidon under concrete, after which it would be left to rot on the ocean floor, far from the reach of other nations, namely China and Russia.Who stole the Poseidon in The Diplomat Season 3? View this post on Instagram Instagram PostKate and Nicol's mutual decision to bury Poseidon under concrete in The Diplomat Season 3 was solid. However, their plan took a different turn when Callum, Kate's secret lover and a British spy, revealed that the nuclear readings of the submarine site had dropped significantly, meaning that somebody had stolen the doomsday weapon. While Kate initially suspected the Russians of theft, she had a chilling realization when she saw Grace and Hal posing together for pictures and conspiring at something.While Grace's husband, Todd, suspected his wife and Hal were involved in an illicit affair, Kate realized that her husband used her to steal the Poseidon from the UK. As she compiles the pieces together, she realizes that the supposedly 'unmanned' drone, sent by Grace to collect pictures of Poseidon, was actually an Ohio-level submarine, big enough to carry the weapon.While Kate was thinking about working on her marriage with Hal in The Diplomat Season 3 finale, this chilling Poseidon reveal turned the tide once again. She confronts Hal with the truth, who does not deny her accusations, rather warns Grace that Kate knows about the weapon's whereabouts.With Poseidon in America's hands, most importantly, in Grace and Hal's hands, the stage is set for The Diplomat Season 4, which will see the changing dynamics between the nations, with Kate stuck in the middle once again.For the latest scoops on your favorite TV shows and movies, follow SoapCentral.