Full story
Two years after Civil War, Scott Lang is days away from finishing the house arrest he got for helping Captain America in Germany. He's on the outs with Hank Pym and Hope van Dyne, who became fugitives because of him. The two briefly open a tunnel to the Quantum Realm, hoping to reach Janet van Dyne, Hank's wife and Hope's mother, lost there since 1987. Scott, who has been to the Quantum Realm before, becomes linked to Janet and gets a message from her in a dream.
Hope and Hank kidnap Scott, leave an ant wearing his ankle monitor at home and put him to work. To finish the tunnel they need a part from black-market dealer Sonny Burch, who quickly sees what Pym's tech is worth and tries to steal it. Hope, now suited up as the Wasp, takes on his men, but a masked figure who phases through walls shows up. It's Ghost, and she makes off with Pym's lab, shrunk to the size of a suitcase.
Hank turns to a former colleague he fell out with years ago, Bill Foster. With his help, the trio finds the lab but walks into Ghost's trap. She is Ava Starr, daughter of Pym's old partner. Her parents died in a quantum experiment that left Ava unstable, her body coming apart and in constant pain. Foster has looked after her ever since. Their plan is to use Janet's quantum energy to cure Ava, which would kill Janet.
The trio escapes with the lab and makes contact with Janet. She gives them her exact location but warns they have two hours before the window closes for a hundred years. Burch gets the location out of Scott's friends with truth serum and tips off the FBI. Scott races home in time to fool Agent Jimmy Woo, but Hank and Hope are arrested, and Ava grabs the lab.
Scott breaks them out. While he and Hope fend off Ava and Burch in a wild chase across San Francisco, with cars shrinking and growing, Hank goes into the Quantum Realm and finds Janet alive. Luis, Dave and Kurt handle Burch's crew. Hank and Janet come back, and Janet willingly gives Ava some of her energy, stabilizing her for now. Ava and Foster vanish. Scott's house arrest ends and he's a free man. In the mid-credits scene, Hank, Hope and Janet send Scott into the Quantum Realm to gather energy for Ava. Before they can pull him out, all three turn to dust. Scott is stuck there, and that is left open.