Quick recap
The Russian submarine Sevastopol sinks, the victim of an AI that has gained a will of its own and slipped into the internet: the Entity. Whoever holds the cruciform key, split into two halves, can control it or destroy it. Ethan gets one half from Ilsa, but thief Grace steals it at Abu Dhabi airport. That's where Gabriel shows up, a man from Ethan's past now working for the Entity. After a chase through Rome, the fight moves to the White Widow's party in Venice, where Gabriel kills Ilsa. On the Orient Express, Gabriel gets the key, but Ethan rides a motorcycle off a cliff and parachutes onto the train. He and Grace escape the cars falling off a bridge, and Ethan gets away with the complete key. The mission continues in the next film.

