In 1989, Hank Pym (Michael Douglas) resigns from SHIELD after realizing that Howard Stark (John Slattery) had been trying to replicate his shrinking technology.
In the present day, Scott Lang (Paul Rudd) has just served his last day at San Quentin State Prison. Waiting to pick him up is his old cellmate, Luis (Michael Peña), who offers to let him stay at his apartment. Scott is unable to find a legitimate job with his criminal record. He had found out that the company he worked for had been overcharging their customers, so he hacked into the system and transferred millions of dollars back to the customers. Luis introduces him to Dave (T.I.), a getaway driver, and Kurt (David Dastmalchian), an identity theft specialist. Luis has a tip about a robbery they could do together.
Hank gets invited to a presentation at the company he founded, Pym Tech. He runs into his daughter, Hope Van Dyne (Evangeline Lilly), and his former protege, Darren Cross (Corey Stoll), who shows him a prototype of the Yellow Jacket suit, modeled after the Ant-Man suit that Hank wore. Darren has not had success at shrinking living tissue.
Scott borrows Luis' van and drops in on the birthday party of his daughter, Cassie (Abby Ryder Fortson). He runs into Cassie's soon-to-be stepfather, Officer Jim Paxton (Bobby Cannavale) , who is not happy to see him. His ex-wife, Maggie (Judy Greer) tells him that he has to find a legitimate job and pay off his delinquent child support before she'll allow visitation with Cassie. Jim and his partner, Gale (Wood Harris) insist that Scott leave the party after saying goodbye to Cassie.
At Pym Tech, Hope and Darren are trying to shrink sheep to microscopic size. Their experiment fails, and a sheep is killed. Darren insists they press forward with another sheep.
Scott realizes that it will be more than a year before he can see Cassie again if he takes a minimum-wage job, so he takes up Luis on his offer. Luis' cousin, Ernesto (Nicholas Barrera) had a friend, Emily (Lyndsi LaRose), who was a housekeeper and was dating Ernesto's friend, Carlos (Carlos Aviles). She told Carlos that the man whose house she was cleaning has a giant safe that's sure to be holding valuables. Scott agrees to help them with the robbery.
Luis picks up supplies for the break-in, and Kurt steals a communication technician's uniform. That night, Kurt climbs up a pole outside Hank's house and blocks the electrical circuit, cutting off all phone communication. Scott climbs the fence and takes out the window sensors, then pries open a window and heads to the basement. He unlocks the basement door only to find a second door that is fingerprint-coded. Using tape, he lifts Hank's print and opens that door. When he gets to the safe, he realizes that he'll need some ingenuity to break the steel door. He drills small holes around the lock, then squirts in water and uses nitrogen to freeze it. The frozen water warps the steel and causes the bolts to shoot out. When he opens the safe, the only thing inside is a weird-looking body suit and helmet. He grabs that and heads out. Meanwhile, Hank has been watching the whole episode upstairs.
Dejected at not having gotten anything valuable, the group heads back to Luis' apartment. Scott goes into the bathroom and examines the suit he stole, wondering why it was so heavily guarded. He notices tubes full of red and blue liquid. Out of curiosity, he puts it on and steps into the bathtub to get a better look at himself in the mirror. Noticing a red button on the glove, he pushes it and instantly shrinks, becoming a half-inch tall. Luis comes into the bathroom and he turns on the water in the tub, which is like a tidal wave for tiny Scott. He hears a voice coming from an earpiece in the helmet. He tries pushing a blue button to return to normal size, but it's broken. Panicked, he jumps away from the water and finds himself sailing across the room. He lands in a crack and falls through to the apartment below. In that room, there's a party going on, and Scott realizes he's on a record player, with the needle moving toward him. He jumps free and lands on the floor where people are dancing. Avoiding the giant footsteps, he gets under the door and into the hallway, where he is promptly sucked up into a vacuum cleaner. When the dust bag gets emptied, he jumps again, this time landing on a rat. He runs away, but lands on a mouse trap. When it's triggered, it sends him flying out through a window and into the rainy night. He sees raindrops as big as his head, before landing on a car. The impact jarred the blue button free, and he grows back to normal size.
Scott races home and takes off the suit. Realizing that someone had been tracking him all along, he decides to return the suit to Hank's safe. He easily breaks into the house again and puts the suit back. But when he gets out, he's surrounded by cops. He is arrested and brought to prison, where Paxton is waiting for him. Gale tells Paxton that Scott's lawyer is there. Confused, Scott goes to see the lawyer that he hadn't asked for and doesn't know.
The "lawyer" waiting for him is Hank. He offers Scott two choices, life in prison or follow his instructions. Scott is thoroughly bewildered, so Hank tells him that he allowed him to steal the suit. Scott is taken back to his cell, and an army of ants brings him back the suit. He puts it on and shrinks, and then quickly escapes from the prison. One of Hank's ants spreads its wings, and Scott climbs on and flies over the city.
Hank tells Scott that he invented the suit, but was afraid it would be misused, and so he had locked it away. Darren had found out about the shrinking technology, but when Hank wouldn't give it to him, Darren forced Hank out of Pym Tech. When Hope realized how dangerous Darren was, she teamed up with Hank to stop him. Hank tells Scott that he needs him to become "Ant-Man" to stop people like Darren. Scott realizes he has a chance to become a true hero, the kind of father his daughter deserves.
Scott gets trained in martial arts and how to lead the ant colonies. He practices his timing with shrinking and growing back. Hank tells him that one piece of technology he'll need to steal the suit from Darren is in an abandoned SHIELD building. When Scott flies to the building, he sees that it's the current Avengers' headquarters. Pressing forward, Scott lands on the roof, triggering a sensor. Sam Wilson (Anthony Mackie) arrives to investigate, and he can see Scott even at small size. A battle ensues, with Scott going inside Falcon's jetpack and causing it to short out. Scott escapes with the equipment he needs.
When Scott returns, Hank explains the truth about how Hope's mother died. There was an ICBM that had been launched at the U.S., and Hank was trying to stop it, but even at ant-size, he couldn't get inside. She had a similar suit, called the Wasp. She decided to shrink to sub-atomic size and was able to stop the missile, but she couldn't return from that size and was lost in the quantum realm.
Darren Cross finally has success at shrinking a lamb. He envisions himself taking over Pym Tech. He invites Hank to the grand unveiling, then calls Hope and tells her that he has increased security, including covering all the vents with microscopic mesh. They realize the job will be harder than they thought, and they'll need more people. Luis, Kurt and Dave are brought in. Luis poses as a security guard and lowers the water pressure so Scott can get in through the pipes. Scott plants explosives throughout the building, but when he tries to steal the suit, he gets caught in a glass cage by Darren, who had anticipated the burglary. Darren plans to sell the technology to HYDRA. Scott breaks free and takes out the Hydra agents. Darren gets into a chopper on the roof and escapes just before the building blows up. Hank and Hope break out by turning a tiny tank to full-size, and ride it out the side of the building.
Darren puts on the Yellow Jacket suit, and he and Scott fight inside the helicopter. After they both fall out, Scott is able to trap Darren inside a bug zapper, but Jim Paxton shows up to arrest him before he can destroy the suit. Darren goes to Jim and Cassie's house and threatens their lives, hoping to draw Scott in. Scott arrives and they fight. The only way Scott can get inside Darren's suit is to shrink to sub-atomic size, and he causes Darren to shrink into nothing. Lang is trapped in the quantum realm, but hearing Cassie's voice, he manages to rewire the suit, reversing the process and returning him to normal size. Following the battle, a grateful Paxton covers for Scott so he would not have to go back to prison. Hank wonders if Hope's mother is also still alive somewhere. Luis tells Scott that the Falcon is looking for him.
Hank shows Hope a new version of the Wasp suit, and tells her that it was made for her. Captain America (Chris Evans) and Falcon have trapped the Winter Soldier (Sebastian Stan), and are trying to figure out what to do next. They decide not to tell Iron Man, and Falcon says he knows who to contact.