In 1989, scientist Hank Pym resigns from S.H.I.E.L.D., which is overseen by Howard and Peggy Carter, upon uncovering their attempt to duplicate his groundbreaking Ant-Man shrinking technology. Deeply concerned about the potential dangers posed by this technology, Pym resolves to keep it hidden for as long as he lives. Meanwhile, [Mitchell Carson] (not linked) attempts to coerce Hank into surrendering his research, but is thwarted by Howard and Peggy.
Pym is the brilliant mind behind the Pym particle, which effectively reduces the space between atoms while simultaneously enhancing density and strength. Fast forward to the present day, and we see Pym's estranged daughter, Hope Van Dyne, alongside his former protégé, Darren Cross, who has taken over Pym Technologies, pushing Pym out of his own company. Cross is on the brink of finalizing his own version of a shrinking suit, dubbed the Yellowjacket. This highly advanced, weaponized version of Pym's original suit poses a significant threat and could have catastrophic outcomes if mass-produced.
Unbeknownst to Darren, Hope is secretly working on behalf of her father, hoping to convince him to give her the original Ant-Man suit to thwart Cross. However, Pym remains firm in his refusal and instead suggests an alternative course of action. Meanwhile, Darren struggles to achieve stable molecular reduction, leading to disastrous results for his organic test subjects, who turn into an unrecognizable mass.
Upon his release from prison, well-meaning thief Scott Lang moves in with his friend Luis and his roommates, Kurt (not linked) and Dave (not linked). Despite holding a master’s degree in electrical engineering, Lang finds himself unable to secure a job due to his criminal past, which forces him to work at a local Baskin-Robbins. When he visits his daughter, [Cassie] (not linked), without prior notice, he faces backlash from his ex-wife Maggie (not linked) and her police-detective fiancé, Paxton (not linked), who urge him to get back on his feet and provide for his daughter.
Determined to make a change, Lang agrees to participate in a burglary orchestrated by Luis. During the break-in, he discovers what he thinks is an old motorcycle suit, but unbeknownst to him, the entire incident is being monitored by Pym through a micro-camera-laden ant. When Lang tries on the suit, he accidentally shrinks down to insect size, leading to a terrifying experience during which he hears Pym’s voice guiding him through his helmet. He returns the suit, but is subsequently arrested.
Pym, who is the suit's original owner, visits Lang in jail, smuggling the suit into his cell to facilitate an escape using the help of ants and insects. Pym has been manipulating events to recruit Lang as the new Ant-Man, hoping he can infiltrate Cross's lab and steal the Yellowjacket. Pym explains his unique connection with ants, revealing how he can communicate with them using electromagnetic waves.
Although Hope is determined to take down Darren, she grapples with her resentment toward Pym for the loss of her mother, Janet, who vanished into the subatomic quantum realm years ago while attempting to disable a Soviet nuclear missile. Pym elaborates that he originally designed the shrinking technology for humans, but deemed it too dangerous and kept it hidden. After some time, Pym's former protégé, Darren, grew suspicious and eventually ousted Pym from his own company.
As they train Lang to master the suit and control the ants, Hope and Pym devise a plan to breach Cross's lab, with the goal to acquire critical data and dismantle Darren's operation. Lang is even sent on a mission to steal a device from the Avengers’ headquarters, where he briefly encounters Sam Wilson, also known as Falcon.
As Darren prepares to unveil the Yellowjacket at Pym Technologies, Lang assembles his crew and a legion of flying ants to disrupt the event by sabotaging servers and planting explosives. However, things spiral out of control when Darren captures Lang, Hope, and Pym, intending to market both the Ant-Man and Yellowjacket suits to Hydra. Lang manages to escape and battles the Hydra agents alongside Hope. Meanwhile, the explosives detonate, causing the building to implode.
With the Yellowjacket suit now operational, Cross tries to eliminate Lang by taking Lang’s daughter, Cassie, hostage. In a desperate move, Lang shrinks down to subatomic size to infiltrate Cross's suit and sabotage it, ultimately leading to Cross's demise. While Lang finds himself lost in the quantum realm momentarily, he successfully navigates back to the macroscopic world.
In the aftermath, Lang receives unexpected support from Paxton, who ensures that he remains free. Pym, now wondering if his long-lost wife is still alive, has a glimmer of hope. In the mid-credits scene, Pym reveals a new Wasp prototype suit to Hope, offering her a chance to inherit the legacy. And in a final post-credits moment, Wilson discusses seeking help from someone with unique abilities, hinting at new adventures to come.
