The Russian submarine Sevastopol is testing its stealth capabilities when it detects another submarine launching torpedoes at it. The captain gives the order to retaliate, but the other submarine suddenly disappears from the detection instruments of the Sevastopol along with its torpedoes. The Sevastopol's torpedo suddenly turns against it, killing the crew. IMF agent Ethan Hunt then receives his next mission to travel to the Arabian Desert to rendezvous with Ilsa Faust, who has a $50M bounty on her head and one half of a key. Ethan fights off the bounty hunters and tells Ilsa to lay low.
Back in the US, Ethan infiltrates a meeting of "the Community", including CIA Director Eugene Kittridge and DNI Denlinger, who are discussing an experimental AI called "the Entity", which was originally designed to infiltrate and sabotage any digital system before erasing all traces, but went rogue and escaped to the Internet. Gaining sentience, the Entity has infiltrated all defense and military systems as well as intelligence networks but without sabotaging anything.
Major powers are now in the race to gain control of the Entity, the source code of which is believed to be in the key. Believing the Entity has become too powerful for humanity to control, Ethan has set out to destroy it. He joins his teammates Luther Stickell and Benji Dunn to Abu Dhabi International Airport to find the holder of the key's second half. Ethan manages to evade the Community's agents, led by Jasper Briggs, as well as a mysterious third party, but the second half is stolen by Grace, a high profile thief. Benji stumbles upon what appears to be a nuclear bomb but succeeds in defusing it after answering a series of riddles and questions from the Entity, however, there is no core for detonation. Rattled by the Entity's intelligence, as well as a ghost from his past named Gabriel, Ethan disbands his team before going after Grace alone.
Following Grace to Rome, Ethan manages to track her down before Briggs and Gabriel close in. After a lengthy chase through the city in which Ethan and Grace are handcuffed together, Grace manages to escape before Ethan rejoins Luther, Benji, and Ilsa. All three agree that this mission is more important than their lives. With support from Benji and Luther, Ethan and Ilsa infiltrate a party hosted by Alanna "The White Widow" Mitsopolis in Venice to find the buyer and to learn what the complete key unlocks.
Once Ethan meets with Grace at the party, they are held at gunpoint by Alanna's guards, and Ethan learns that Alanna has sided with Gabriel, who seems to be desperate to deliver the key to the Entity. Gabriel also gives Ethan a choice: Either save Grace, or save Ilsa, as one of them will die tonight. Ethan fights his way out, eventually subduing Paris, Gabriel's henchwoman, before sparing her. As Ethan is being misguided by the Entity masquerading as Benji, the team splits up while Gabriel fights both Grace and Ilsa, in which the latter sacrifices herself for the former moments before Ethan could intervene.
Remorseful, Grace joins the team to board the train where Alanna will meet with the buyer and to track the key down. Moments before going off the grid to investigate traces of the Entity, Luther warns Ethan that he must not kill Gabriel since he knows what the keys are for, and Ethan killing Gabriel or Ethan dying on the train are the two outcomes the Entity has predicted once Ethan boards the train, but regardless of what will happen, the Entity will win. On the train, Gabriel kills the conductor before destroying the controls and setting it to maximum speed. He then meets with Denlinger, who proposes to make an alliance between him, Gabriel, and the Entity, and explaining the complete key will unlock the chamber that houses Sevastopol's computer.
It's revealed that an early system of the Entity was injected on the system to sabotage the sub's stealth capability, causing its destruction. Denlinger claims that the early version and its source code are still in the sub's computer chamber, and whoever can access it can control or destroy the Entity. After ensuring Denlinger only knows about the Sevastopol, Gabriel kills him before wounding Paris, knowing that she will betray him due to Ethan sparing her during their fight in Venice. While impersonating Alanna, Grace meets with the buyer, revealed to be none other than Kittridge, and brings him the complete key. While tempted to betray Ethan over $100 million, she instead takes the key back and flees.
Ethan parachutes off a cliff and into the train, saving Grace, but Gabriel steals the key. Ethan fights Gabriel for the key, eventually subduing him, but remembers his earlier promise to Luther that he cannot kill him since he knows the secrets of the key and is the only lead to the lock. When Briggs and Degas intervene, Gabriel escapes with the key seemingly in his possession in the ensuing chaos before initiating a countdown to detonate a bridge ahead. Ethan convinces Briggs to save the passengers, while Ethan and Grace detach the locomotive before being rescued by Paris and escaping the front carriages plunging down the broken bridge. Moments before passing out, Paris informs Ethan about the Sevastopol. Grace tells Kittridge that she has decided to join the IMF, while Ethan escapes from the train and reunites with Benji. As Gabriel realizes he has escaped with empty pockets, Ethan drives off with Benji with the key in his possession.
End Part One
