The movie opens with Hank Pym, Peggy Carter and Howard Stark discussing the potential dangers of the Ant-Man technology.
Hank and Janet Van Dyne operate as Ant-Man and the Wasp for SHIELD during the Cold War. During one mission, they need to prevent a nuclear warhead from hitting a populated area. Hank's suit is damaged, so Janet shrinks beyond the suit's capacity to defuse the bomb and disappears, believed to be dead. Hank believes the technology is too dangerous and seals it away.
Lang is forced to infiltrate the New Avengers' headquarters to retrieve an energy source developed by Stark Industries that they need to complete their heist on Pym Technologies, and comes across the Falcon. They fight, and Lang escapes with the device after disabling the Falcon's propulsion device. The Falcon then begins searching for Ant-Man.
At one point, the Falcon visits one of his contacts that is monitoring superhero activity. She mentions reports of a man who can climb walls and swing through buildings, but no word on a shrinking man.
In the final battle, Lang is also forced to shrink beyond the suit's capacity and ends up in the Microverse, but manages to return. It's implied Janet is lost there after the accident in the 1980's.
Cross dies after Lang damages the Yellowjacket's Pym Particles supply, causing Cross body to shrink and expand in gruesome ways. Lang also accidentally creates a giant-sized ant that becomes Cassie's pet.
Movie ends with Falcon finding Ant-Man to discuss things.
Mid-credits scene is Pym giving Hope a modified version of the Wasp suit.
After-credits scene is Captain America and Falcon finding a beat-up War Machine being held captive in a fortified base. After Captain America declines to calling Iron Man for help, Falcon decides to call Ant-Man.