Ethan Hunt (Tom Cruise) and his IMF team, Benji (Simon Pegg), and Luther (Ving Rhames), are tasked with bringing down a terrorist who goes by the name of John Lark, and who works with 12 people who call themselves "The Apostles," in order to blow up major cities around the world, using nuclear power.
Alan Hunley (Alec Baldwin), who left the CIA and is now the head of the IMF, introduces Ethan to a CIA agent named August Walker (Henry Cavill), who's been ordered by his boss, Erica Sloan (Angela Bassett), to keep a close eye on everything Ethan does — because the CIA still worries that he's gone rogue. When Ethan and August go to find Lark and stop him, a number of things go wrong, but luckily they run into Ilsa Faust (Rebecca Ferguson), who again helps Ethan with his mission.
With Ilsa's help, Ethan manages to pose as Lark in an effort to meet White Widow (Vanessa Kirby) in order to find the three spheres of plutonium Lark plans to use to commit major genecide.
A young Christopher Robin has a going away party in the Hundred Acre Wood to say goodbye to his stuffed animal friends – Winnie-the-Pooh, Tigger, Piglet, Rabbit, Roo, Kanga, Eeyore and Owl. He promises them all he will never forget them as he heads off for boarding school, but as an adult, Christopher Robin (Ewan McGregor) is much too busy to think about his friends. In fact, he barely has time to think about his wife Evelyn (Hayley Atwell) or daughter Madeline (Bronte Carmichael), because he's far too busy at work.
The luggage company he works for is facing severe cutbacks, so Christopher Robin has to work on the weekend instead of going to the cottage with his family as he promised, in an effort to save his co-workers' jobs and help get the company successful again.
Meanwhile, Winnie-the-Pooh can't find any of his friends. He searches Christopher Robin out in a park and appeals to him for help. Although he's consumed by his work, Christopher still has a soft spot in his heart for Pooh and eventually agrees to help. He takes the bear by train back to the Hundred Acre Wood again, but finds it dark and gloomy and not at all as he remembers it.