Struggling to rebuild his life after being released from prison, skilled thief Scott Lang is stuck in a cycle of unemployment and estrangement from his beloved daughter, Cassie. Desperate to prove himself responsible and regain visitation rights, Scott reluctantly agrees to a seemingly simple burglary at the behest of his friend Luis. Unbeknownst to Scott, the heist is orchestrated by Dr. Hank Pym, a brilliant scientist and the inventor of the Pym Particle—a substance that allows objects to shrink or grow in size. When Scott burgles Pym's home, he finds not riches but a suit that grants him the ability to shrink to insect size while increasing his strength.

As Scott experiments with the suit and slowly masters its capabilities, Dr. Pym reveals his true intentions. He has been watching Scott and now enlists him for a critical mission. Pym's former protégé, Darren Cross, has replicated his size-altering technology, creating the potentially dangerous Yellowjacket suit. A menacing figure who has forced Pym out of his own company, Cross intends to sell the technology as a weapon, prompting Pym to take action to thwart his plans. Scott is to use his unique skills—theft techniques honed over years and the new abilities provided by the Ant-Man suit—to infiltrate Pym Technologies and stop Cross before the technology falls into the wrong hands.