A next-generation Russian stealth submarine Sevastopol employs an advanced AI, activated by a two-piece cruciform key. The submarine is under Bering strait, and the Russian Navy is testing the AI system to help evade enemy submarines. So far, the system has performed miraculously, and the submarine has deliberately approached every navy in the world and remained undetected. The AI system is enclosed within a fortified done housed within the submarine. The AI deceives the crew into attacking a phantom target only to be struck by their own torpedo, killing all aboard. The flooded submarine, along with the fortified dome housing the AI system sits at the bottom of the Bering strait. The bodies of the 2 officers with the 2 parts of the key, floats to the surface.Ethan Hunt is assigned the task to retrieve the key, one half of which is with Isla Faust. IMF agent Ethan Hunt (Tom Cruise) travels to the Empty Quarter of the Arabian Desert and successfully retrieves a key piece from disavowed MI6 agent Ilsa Faust (Rebecca Ferguson). He also fakes her death to ensure she survives a bounty of $50 million placed by an unknown source.Back in Washington, D.C., he infiltrates a U.S. Intelligence Community briefing for Director of National Intelligence Denlinger (Cary Elwes) discussing the AI, known as the "Entity" (Simon Pegg). CIA director Eugene Kittridge (Henry Czerny) states the Entity achieved sentience and can manipulate cyberspace, allowing it to control global defense intelligence and financial networks. It has infiltrated all major defense systems globally and has not done anything. It sent a very clear message that it can return anytime and assume control. The CIA does not know the goal of the Entity. The Entity now resides in the cloud. Now it is infiltrating all intelligence networks around the world and corrupting their data. Governments are taking physical backups of their files before they are lost to the Entity. The Entity would know every weakness of every country, network, system out there. The servers have been isolated from the outside world, but the Entity has managed to bribe and blackmail humans to infiltrate the servers and continue towards its goal.World powers compete to obtain the cruciform key to control the Entity, though the exact means of controlling it are unknown. Kittridge says that a buyer is passing through the Middle East in the next 72 hours and already has one half of the key. Kittridge tells Denlinger that the other half of the Key was with Ilsa, and he had sent his agent to retrieve the key from her.Revealing himself, Ethan converses with Kittridge and deduces that Kittridge placed Ilsa's bounty. Ethen refuses to give the half key to Kittridge and states his intention to destroy the Entity, knowing they would be considered rogue actors and are participating in a global race. Ethan and his IMF teammates Benji Dunn (Simon Pegg) and Luther Stickell (Ving Rhames) travel to Abu Dhabi International Airport to intercept the buyer of the other key piece while evading US agents led by Jasper Briggs (Shea Whigham). Ethan's plan was to sell their half of the key to the buyer, and then follow the buyer to figure out what it unlocks. Ethan locates the buyer at Abu Dhabi airport and follows him to his flight to Venice.During the pursuit, the key piece is stolen by a professional thief named Grace (Hayley Atwell). Ethan confronts Grace and offers to put the key back in the buyer's pocket in exchange for a generous payment of $4 million. But the buyer was already dead and the key that Grace stole was a counterfeit. Which means that someone had manipulated Ethan to be at the airport with the real key. Concurrently, Luther and Benji disarm a hoax nuclear device, which was headed to the same flight as the buyer and Ethan. While Ethan was distracted, Grace steals the real key from Ethan. Ethan suspects foul play after seeing Gabriel (Esai Morales), a sadistic Entity liaison with ties to his Pre-IMF past. Gabriel is a powerful terrorist and Ethan's adversary who appears to be working with the Entity. He and Ethan had a fateful encounter with each other prior to Ethan becoming an IMF agent.Aborting the mission, the team scatters and Grace escapes to Rome. She is apprehended upon arrival but Ethan rescues her from local authorities, US agents, and an Entity operative named Paris (Pom Klementieff). Paris pursues Ethan and Grace and kills many US agents and Roman police along the way, but they still manage to escape by working together. Grace had revealed to Ethan that she was delivering the key to a party in Venice. Grace escapes again, while Ethan rendezvous with Luther, Benji, with Ilsa rejoining them. With Benji and Luther providing support, Ethan and Ilsa follow Grace to Venice where they infiltrate a party held by the arms dealer Alanna Mitsopolis (Vanessa Kirby), also known as the White Widow. Luther had figured out by analyzing the video feed from the airport that Gabriel is working for the Entity and was most probably the person who planted the bomb. Ilsa says that Gabriel is the Entity's messenger and that the key leads to the Entity's source code. But it was Ilsa's friends at MI6 who shared this information with her digitally. Ethan suspects that the Entity is manipulating their actions and there is a reason why it has assembled all his dear friends in Venice. But the 4 agree that they cannot let Gabriel have the complete key.All groups, Alanna, Gabriel, Ethan, Grace, Ilsa gather at the party. Ethan realizes that the party was organized by the Entity as well. All groups disclose their role in acquiring the complete key. Grace was hired by Alanna to steal the other key piece to produce the completed key, which will be sold to her buyer the next day on the Orient Express. Ethan knows that Alanna doesn't want to keep the key for herself as she does not know what it unlocks. Alanna cannot decide whether to give the key to her friend Ethan, or to Gabriel and put her fate in his hands.Through the Entity, Gabriel proclaims he will possess the completed key the next day and that either Ilsa or Grace will die. Ethan unsuccessfully attempts to dissuade Alanna from the sale, allowing Gabriel and Grace to escape. Ethan pursues Grace (as she still has one half of the key), but the Entity hacks into their communications and impersonates Benji, leading him away from Grace and into a fight with Paris, whom he spares.Concurrently, The Entity leads Grace and Isla to Gabriel. Gabriel incapacitates Grace and kills Ilsa, devastating Grace and Ethan, who vows vengeance. Thus, Grace agrees to impersonate Alanna and take the key during the sale. Luther leaves for an off-grid location to prevent interference from the Entity, advising Ethan to spare Gabriel to determine information about the Entity. Luther says that the Entity knows that Ethan is the only one who wants to kill the Entity, which is why it used Gabriel to kill Ilsa, so Ethan would be provoked to kill Gabriel and with him all knowledge about what the key unlocks.The next day, on the train, Gabriel kills the engine crew and destroys the throttle and brake. Grace infiltrates the train impersonating Alanna and gets her half of the key. With Paris, Gabriel meets Denlinger, who divulges information only he knows in an attempt to form an alliance between himself and the Entity.Originally an advanced cyber weapon developed by the US, the Entity went rogue following its deployment against the Sevastopol (The US deployed an early copy of the on the submarine to sabotage her stealth capability). Meanwhile, outside, the AI evolved and rewrote itself, becoming the Entity. So, inside the submarine is the only remaining copy of the AI's original source code which can be used to control or destroy it. The completed key unlocks a chamber inside the vessel containing the Entity's source code, allowing it to be destroyed or controlled. Gabriel subsequently kills Denlinger and tries to kill Paris as the Entity determined she would betray them after Ethan spared her life. Impersonating Alanna, Grace brings the key to Kittridge, revealed to be the buyer, and negotiates a $100 million sale alongside protection for herself, but pickpockets the key from Kittridge after canceling the transfer. Ethan parachutes into the train to save Grace, but Gabriel acquires the key.Ethan and Gabriel fight atop the train, but Gabriel escapes and detonates a bridge ahead. Grace and Ethan detach the locomotive from the rest of the train, saving the passengers. Paris rescues both from falling and manages to tell Ethan the key's connection to the Sevastopol before losing consciousness. Grace informs Kittridge of her desire to join the IMF.Ethan flees the wreckage by para-glider with the completed key, which he took from Gabriel during the fight, and rendezvous with Benji to continue the mission, which is finding the Sevastopol and destroying the Entity.