Armed with a super-suit with the astonishing ability to shrink in scale but increase in strength, con-man Scott Lang must embrace his inner hero and help his mentor, Dr. Hank Pym, plan and pull off a heist that will save the world.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, and then stole five million for himself. 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. In the room with them is an ant with a video camera.
In a restaurant, Hope Van Dyne (Evangeline Lilly) is sitting with Darren Cross (Corey Stoll), as he reminisces about their jobs working for Hank Pym (Michael Douglas).
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, 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, and then notices several ants, all looking at him.
Scott goes to the house, which belongs to Hank, jumps the fence, and cuts the cable line. He knocks on the door, posing as a cable repairman. Hank lets him in, and he plants a camera in the cable box. Hank presents himself as a perfect mark for 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.