In 1989, scientist Hank Pym resigns from S.H.I.E.L.D. after discovering that they attempted to replicate his Ant-Man shrinking technology. Believing the technology is dangerous, Pym vows to hide it as long as he lives. In the present day, Pym’s estranged daughter, Hope van Dyne, and former protégé, Darren Cross, have forced him out of his own company. Cross is close to perfecting a shrinking suit of his own, the Yellowjacket, which horrifies Pym.
Upon his release from prison, well-meaning thief Scott Lang moves in with his old cellmate, Luis. Lang’s ex-wife, Maggie—engaged to policeman Paxton—agrees to let Lang see his daughter Cassie if he provides child support. Unable to hold a job because of his criminal record, Lang agrees to join Luis’ crew and commit a burglary for money. Lang breaks into a house and cracks its safe, but only finds what he believes to be an old motorcycle suit, which he takes home. After trying the suit on, Lang accidentally shrinks himself to the size of an insect. Terrified by the experience, he returns the suit to the house, but is arrested on the way out. Pym, the homeowner, visits Lang in jail and smuggles the suit into his cell to help him break out.
At his home, Pym, who manipulated Lang through Luis into stealing the suit as a test, wants Lang to become the new Ant-Man to steal the Yellowjacket from Cross. Van Dyne, who has been spying on Cross for Pym despite her strained relationship with her father, helps Pym train Lang to fight and to control ants. They send him to steal a device from the Avengers’ headquarters, where he briefly fights Sam Wilson. While van Dyne still shows anger towards Pym about her mother Janet’s death, he reveals that Janet, known as the Wasp, disappeared into a subatomic quantum realm to disable a Soviet nuclear missile. Pym warns Lang that he could suffer a similar fate if he overrides his suit’s regulator.
Cross perfects the Yellowjacket and hosts an unveiling ceremony at his headquarters. Lang, along with his crew and a swarm of flying ants, infiltrates the building during the event, sabotages the company’s servers, and plants explosives. When he attempts to steal the Yellowjacket, he is captured by Cross, who intends to sell both the Yellowjacket and Ant-Man suits to Hydra, led by Mitchell Carson. Lang breaks free and dispatches most of the Hydra agents, though Carson is able to flee with a vial of Cross’ particles. Lang pursues Cross as he escapes, while the explosives detonate, imploding the building.
Cross dons the Yellowjacket and attacks Lang before Lang is arrested by Paxton. His mind addled by the imperfect shrinking technology, Cross takes Cassie hostage to lure Lang into another fight. Lang overrides the regulator and shrinks to subatomic size to penetrate Cross’ suit and sabotage it to shrink uncontrollably, killing Cross. Lang disappears into the quantum realm but manages to reverse the effects and returns to the macroscopic world. In gratitude for Lang’s heroism, Paxton covers for Lang to keep him out of prison. Seeing that Lang survived and returned from the quantum realm, Pym wonders if his wife is alive as well. Later, Lang meets up with Luis, who tells him that Wilson is looking for him.
In a mid-credits scene, Pym shows van Dyne a new Wasp prototype suit and offers it to her. In a post-credits scene, Wilson and Steve Rogers have Bucky Barnes in their custody. Unable to contact Tony Stark because of “the accords”, Wilson mentions that he “know[s] a guy”.
(All credit to Wikipedia)