Scott Lang is getting out of prison. A master burglar with a Robin Hood streak—he broke into a corrupt corporation and returned money to the people they'd cheated—Scott just wants to go straight for his daughter Cassie. But with a criminal record, even a degree in electrical engineering can't get him a job. His ex-wife Maggie and her new husband, cop Paxton, won't let him see Cassie until he proves he can provide for her.
Desperate, Scott agrees to one last heist with his old crew: Luis, Dave, and Kurt. Their target is a mysterious safe in a wealthy recluse's mansion. After an impressive break-in, Scott cracks the safe and finds... a strange leather suit. Disappointed, he takes it anyway.
The suit belongs to Dr. Hank Pym, the original Ant-Man, who shrunk down to fight alongside S.H.I.E.L.D. in the Cold War until his wife Janet was lost to the Quantum Realm—a subatomic dimension between atoms. Hank quit S.H.I.E.L.D. and has spent decades protecting his Pym Particles, the technology that enables shrinking, from those who would weaponize it.
Now his former protégé Darren Cross has taken over Pym's company and is close to replicating the technology as "Yellowjacket," a weaponized shrinking suit he plans to sell to HYDRA. Hank needs someone to break in and destroy the research. He needs a thief. And he's been watching Scott, engineering the heist to test him.
Hank's daughter Hope is skeptical of Scott. She's trained in combat and understands the technology—she should wear the suit. But Hank refuses to risk losing her the way he lost Janet. Hope resents being sidelined and initially despises Scott, but she agrees to train him.
Scott learns to control the suit's shrinking and growing capabilities, communicate with ants using an earpiece, and fight at shifting scales. The training montage is equal parts science lesson and comedy as Scott struggles with the disorientation of sudden size changes. His relationship with Hope evolves from antagonism to respect to something more.
The heist requires Scott to infiltrate the Avengers' new compound to steal a signal decoy. He encounters Falcon and they fight—a tiny man versus a flying superhero. Scott wins through creativity and escapes, cementing his credentials. Sam Wilson immediately starts looking for "the guy who can get small."
The final heist goes wrong when Cross reveals he knows they're coming. He's been manipulating Hank, waiting for this moment to take the Ant-Man suit as well. Cross dons the Yellowjacket armor and proves to be a formidable opponent—the Pym Particles have subtly affected his mind, making him paranoid and ruthless.
The battle escalates across San Francisco and eventually into Cassie's bedroom, where Cross threatens the little girl. Desperate, Scott does what Hank said was impossible: he goes subatomic, shrinking between the molecules of Cross's suit to destroy it from within. Cross is crushed out of existence.
But Scott keeps shrinking, falling into the Quantum Realm—the dimension that claimed Janet. Time and space lose meaning. Following Cassie's voice, Scott manages to trigger his suit's growth mechanism and returns to the normal world. The fact that he survived gives Hank hope that Janet might still be alive.
Scott becomes a hero, and Paxton covers for him with the authorities. He finally gets to be part of Cassie's life. Hank shows Hope a new prototype suit: the Wasp, which Janet once wore alongside him. "It's about damn time," Hope says.
In a mid-credits scene, Hank presents Hope with the completed Wasp suit. In a post-credits scene, Sam and Steve have located Bucky Barnes, but can't involve Tony because of "the Accords." Sam knows a guy who can help—the guy who can get small.