Returning from another mission, Ant-Man dwells on his loneliness and the loss of his late wife: Maria Pym. Flashing back to the past we learn that on their honeymoon, Henry and Maria Pym visited Maria's native Hungary, in spite of Henry's fears that her history as a political prisoner would make her a target. Shortly upon arrival in Hungary, Maria is captured by armed men and Henry is knocked out by the butt of one of their guns. Recovering at the American Embassy, Pym is informed that his wife has been murdered, as a lesson to those who wish to leave the Iron Curtain. Angered, Pym attempts to track down his wife's killers but only lands himself in jail. He is freed by Embassy officials and sent home. There, Pym uses his late wife's old saying "Go to the ants, thou sluggard!" as the inspiration for devising his size-changing formula and becoming Ant-Man.
His reflection over, Ant-Man realizes that he cannot continue his crusade against crime alone and needs a partner to aid him in his work. He begins working on a new set of abilities for his future partner utilizing wasp biology, but is interrupted by the arrival of Professor Vernon van Dyne and his young daughter, Janet.
Pym immediately realizes that Janet bears a striking resemblance to his late wife, Maria, but dismisses any attraction due to her age and apparent ditzy personality. Meanwhile, Janet also dismisses any attraction to him, due to the stereotype that scientists are boring individuals. Vernon asks Pym to look at his Gamma Ray Beam device which would allow him to make contact with another galaxy. While Pym finds it interesting, he explains to van Dyne that his area of expertise is in biology, not astronomy. Realizing Pym isn't interested in the project, van Dyne and his daughter leave.
Later that day, van Dyne attempts to use his Gamma ray beam and inadvertently transports a creature from the planet Kosmos. The creature states that it is an escaped criminal. It uses its powers to kill Dr. van Dyne and then flees. Janet stumbles upon her father's body. In a panic she calls Henry Pym, who dismisses her once again as a bored ditzy girl and hangs up, but later learns his error when he finds out the truth from his ants.
Donning his Ant-Man identity, he travels to the van Dyne house and finds that Professor van Dyne was killed by pure fear. He tells Janet to report her father's death to F.B.I. agent Lee Kearns, and tells her to go see Henry Pym right after. On his way back to his lab, Ant-Man learns from his ants that there were large traces of an alien form of formic acid, which frightens his ants.
Janet van Dyne arrives at Dr. Pym's lab, where Henry reveals to her his identity as Ant-Man and asks her to become his sidekick, the Wasp. Agreeing, Janet is implanted with wasp wings and antenna that she can use while shrunken down by Ant-Man's shrinking gas. As Ant-Man and Wasp, the duo arrive on scene where the creature from Kosmos is on a rampage. Before going into battle, Janet announces that she's in love with Hank, which he immediately dismisses, due to her age and his fear of a second tragedy in his love life. Even with the aid of the military, and Ant-Man's army of ants they are unable to stop the monster's rampage.
Realizing what he must do, Ant-Man and Wasp return to base where Ant-Man devises a compound to counteract formic acid. He then puts the compound into shotgun shells and, with the aid of his ants, succeeds in dispersing the creature by shooting it with a number of shells of the compound.
With the monster defeated, Ant-Man and Wasp return home, where Ant-Man calls F.B.I. agent Kearns to tell him that the menace had ended. When Kearns asks to be Ant-Man's partner, Ant-Man declines, telling the F.B.I. agent that he already has a partner. Janet thinks to herself that she'll certainly soon make him fall in love with her, and will stay with him so that he does.