In 1989, brilliant scientist Hank Pym resigns from S.H.I.E.L.D. after discovering that Howard Stark and other officials are trying to duplicate his revolutionary Pym Particle technology. Because the particles can shrink objects and people while dramatically increasing their density and strength, Hank believes the invention is too dangerous to be weaponized. Determined to keep it out of the wrong hands, he walks away from the organization and vows to protect the secret himself.
Years later, Hank has grown older and become estranged from his daughter, Hope van Dyne. He is also pushed out of his own company, Pym Technologies, by his ambitious former protégé Darren Cross. Cross has been studying Hank’s old research and is close to perfecting a militarized shrinking suit called Yellowjacket. Realizing that Cross intends to sell the technology to criminals and hostile organizations, Hank knows he must act quickly before the weapon changes the balance of power around the world.
At the same time, Scott Lang is released from San Quentin State Prison after serving three years for burglary. Although Scott originally broke into a corrupt corporation to expose financial fraud and return stolen money to customers, his criminal record makes rebuilding his life nearly impossible. He wants nothing more than to support his young daughter Cassie and become a better father, but he cannot find steady work. His ex-wife Maggie and her fiancé, police officer Jim Paxton, warn Scott that he will not be allowed to see Cassie unless he begins paying child support.
Desperate and out of options, Scott reluctantly agrees to join a burglary suggested by his old friend Luis. The target appears to be an elderly man’s home with a valuable safe inside. Scott skillfully breaks in and cracks the safe, only to find what looks like an old motorcycle suit. He takes the strange suit home, disappointed but curious. When he puts it on and activates the controls, he suddenly shrinks to the size of an insect. Panicked, Scott struggles to survive in a now gigantic world before finally returning to normal size.
Frightened by the experience, Scott tries to return the suit to the house, but the police arrest him. In jail, he is visited by a lawyer who is secretly Hank Pym in disguise. Hank smuggles the suit into Scott’s cell and instructs him to escape. With the help of ants responding to hidden technology in the helmet, Scott breaks out and is brought to Hank’s home. There, Hank reveals that the burglary was a test. He selected Scott because of his intelligence, agility, and criminal expertise, believing Scott is the perfect person to become the new Ant-Man.
Scott agrees to help stop Darren Cross. Hank and Hope begin training him in combat, stealth, and mastering the suit’s controls. Hope proves to be a strict but highly capable teacher, though she resents her father for choosing Scott over her. Scott also learns to command different species of ants for transportation, communication, and tactical support. Though the training is difficult, Scott gradually embraces the responsibility of becoming a hero.
To prepare for the mission, Hank sends Scott to retrieve a signal-jamming device from an old Stark warehouse that is now part of the Avengers Compound. During the infiltration, Scott encounters Sam Wilson, also known as Falcon. The two engage in a brief fight, and Scott barely escapes with the device. The incident later leads Falcon to begin searching for Scott.
Meanwhile, tensions between Hank and Hope finally erupt. Hank admits the truth about Hope’s mother, Janet van Dyne. Janet was Hank’s partner, the original Wasp, who sacrificed herself years earlier by shrinking beyond the atomic level to disable a Soviet missile. She vanished into the mysterious Quantum Realm and was presumed dead. Hank confesses he refused to let Hope wear the Ant-Man suit because he feared losing her the same way. Father and daughter reconcile, and Hope fully joins the mission.
As Cross prepares a public unveiling of Yellowjacket, Hank’s team launches their heist. Scott sneaks into Pym Technologies with the help of Luis, Kurt, Dave, and an army of ants. They sabotage the building and plant explosives meant to destroy Cross’s research. However, Cross anticipated the attack and captures Scott. He reveals that he has already arranged buyers for the Yellowjacket technology, including Hydra agents and members of the Ten Rings.
Chaos erupts when Scott escapes and attacks the buyers. Hope and Hank join the battle while the planted explosives destabilize the facility. Amid the confusion, former S.H.I.E.L.D. agent Mitchell Carson escapes with samples of the shrinking formula. Cross dons the completed Yellowjacket suit and flees, determined to get revenge. The explosions collapse the Pym Technologies building in a catastrophic implosion.
Cross tracks Scott to Cassie’s home and kidnaps her. Scott confronts him in a final battle inside Cassie’s bedroom, where miniature combat turns toys, electronics, and household objects into enormous obstacles. Scott eventually realizes he cannot defeat Cross conventionally. In a desperate gamble, he damages his own regulator and shrinks beyond safe limits into the Quantum Realm, entering Yellowjacket’s suit circuitry and destroying it from within. Cross loses control and implodes.
Lost in the strange subatomic dimension, Scott nearly gives up, but memories of Cassie inspire him to survive. He uses a growth disc on his regulator and miraculously returns to normal size. Hank is astonished, believing no one could ever escape the Quantum Realm. Scott’s survival gives Hank hope that Janet may still be alive somewhere within it.
After witnessing Scott’s heroism, Paxton clears him of wrongdoing and accepts him as part of Cassie’s life. Scott reunites with his daughter and embraces his new identity as Ant-Man. Later, Luis tells Scott that someone is looking for him. Sam Wilson leads him to Captain America and the Avengers, while Hank reveals a prototype Wasp suit to Hope, finally inviting her to continue her mother’s legacy.
