Full story
In 1989, scientist Hank Pym finds out S.H.I.E.L.D. tried to copy his shrinking formula, the Pym Particle, and quits, swearing no one will ever get it. Years later, his daughter, Hope van Dyne, and his former protégé, Darren Cross, push him out of his own company. Cross is close to recreating the tech in a military suit called the Yellowjacket.
Scott Lang gets out of San Quentin. He was an engineer who turned thief when he exposed a scam at the company he worked for. He wants to go straight and see his daughter, Cassie, but his ex-wife, Maggie, and her fiancé, cop Jim Paxton, want him to pay child support and find a place first. Nobody will hire him. His motormouth friend Luis offers one last job: a safe in a mansion. Scott cracks it and finds only an old motorcycle suit.
At home he puts on the suit, presses a button and shrinks to the size of an ant. Freaked out, he returns everything and gets arrested. It turns out Pym set up the whole burglary to test him. He breaks Scott out of jail and offers a deal: Scott becomes the new Ant-Man and steals the Yellowjacket before Cross can sell it. Hope, working undercover with Cross, hates watching her father pick someone else over her. The two train Scott to fight, shrink and control ants. Pym explains why: Hope's mother, Janet, the original Wasp, shrank too far on a mission in 1987 and was lost in the Quantum Realm.
To grab a piece of equipment, Scott breaks into an Avengers facility and tangles with Sam Wilson, the Falcon, and gets away. At the Yellowjacket unveiling, Scott, Luis, Dave and Kurt slip into Pym Tech with a swarm of ants to wreck the servers and plant explosives. Cross catches on, reveals he is selling the suits to Hydra and grabs the three of them. Scott and Hope break free and the building blows, but a Hydra agent escapes with a vial of particles and Pym is shot.
Cross suits up as Yellowjacket and fights Scott inside a briefcase and then on a toy train set. Paxton arrests Scott, and Cross takes Cassie hostage. Scott gets away, reaches the villain in his daughter's bedroom and, out of options, switches off his suit's regulator. He goes subatomic, slips inside Cross's armor and wrecks it. Cross shrinks into nothing. Scott finds his way back out of the Quantum Realm using an enlarging disc. Paxton covers for him with the police, and Pym, seeing that Scott came back, starts to believe Janet might be alive. In the closing scenes, Pym shows Hope a new Wasp suit, and Steve Rogers and Sam Wilson have Bucky Barnes in custody. Sam says he knows a guy who can help.