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.
Darren Cross finally has success at shrinking a lamb. He envisions himself taking over Pym Tech.
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 wonders where Scott went. Luis turns on the water in the tub, which is like a tidal wave for tiny Scott. 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. He realizes that he now has a super power.
Scott spends several days fixing up the suit, and then uses it to slip inside an ATM and rewire it to spit out all its money. He goes to Maggie's house and pays up all the child support he owes, plus the next year. When Luis, Dave and Kurt go to a casino, Scott shrinks himself and hides on the van. When they are playing craps, Scott would secretly turn the dice to make them win. He also uses the suit to see Cassie at school. When a bully tries to bean her in a game of dodgeball, he grabs the ball and causes it to whack back into the bully's face. His next plan is to rig the lottery by grabbing the balls he needs and causing them to be sucked up into the chute. This plan fails when he's unable to get the last ball in time. In Paxton's office, he uses a rubber band launcher to fling paper clips at Paxton. Seeing an armored truck outside of a bank, he realizes he could rob it without anyone getting hurt.
He goes to see Cassie at her home, and she is getting ready for bed. Cassie tells Maggie that she heard people say Scott is a bad man. Scott hears a voice ask him if he really is a bad man. He realizes that the voice is coming from an earpiece inside the helmet. He races home and returns to normal size, then 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 didn'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. Hope is Hank's daughter, and when she 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.