Hank Pym created the Ant-Man technology in the 1970's and operated as a spy for S.H.I.E.L.D. throughout the Cold War, answering to Howard Stark and Peggy Carter. His wife, Janet Van Dyne, was the Wasp and apparently died in one of his experiments, and he blamed himself, leaving the organization to build his own company, Pym Technologies, and refusing to become Ant-Man once again. Although Janet is believed to be dead, there are indications she could be alive, lost somewhere in the Microverse.
Hope Van Dyne is Pym's daughter and the head scientist of Pym Technologies, as well as a member of the Board of Directors. She blames him for her mother's death and conspired to oust him from the company. She is a uptight, humorless woman and is infuriated when Pym chooses Scott Lang over her to become the Ant-Man, though she begrudingly agrees to train him.
Darren Cross is Pym's protege who wrests control of Pym Technologies once Pym refushes to share the secrets of the Ant-Man technology due to his fear of it being weaponized and acquired by third parties. Cross is obsessed with earning Pym's approval and carving his name in History, and believes that he can accomplish that through the Yellowjacket armor. Cross grows increasingly deranged as the movie progresses, and has several shades to him, ranging from charming to menacing to extremely goofy.
Pym can no longer use the suit as excessive use causes debilitating physical and psychological side effects.
Scott Lang is a thief specialized in navigating advanced security systems to pull off heists. After being arrested for stealing money that had previously been emblezzed by a former employer to return it to its rightful owners, keeping a share for himself, Lang serves five years in prison before being released. In his absence, his ex-wife Maggie has married policeman Jim Paxton, a childhood friend of Lang's. Maggie and Paxton forbid Lang from interacting with his daughter Cassie due to his criminal background and delayed alimony payments, so Lang is forced into crime once again to gather enough money to be allowed to be close to Cassie.
In prison, Lang befriends his cellmate, con man Luis, who is released beforehand. When Lang is released, he begins sharing an apartment with Luis and join him and his accomplices, identity theft specialist Kurt and getaway driver Dave, to rob Pym's house. Lang successfully cracks the safe, but finds only the Ant-Man suit, which he steals, mistakenly assuming it's a motorcycle suit. Returning to his apartment, he decides to put it on, and notices a button, which shrinks him upon being pressed. Luis accidentally flushes Lang down the drain, damaging the button and preventing Lang from returning to normal size. Lang then navigates the building's pumbling before falling into a dance club below, where he avoids being stomped by the dancing patrons before finally repairing the suit and returning to normal size.
Pym engineered the theft to test Lang's abilities in evading his state-of-the-art security system, and to evaluate his character by monitoring what he would do with the suit. When Lang is arrested breaking into Pym's house to return the suit, Pym bails him out and extends an offer for him to become the Ant-Man.
Lang makes modifications to the suit throughout the movie.