Plot

Scott Lang and Hope Van Dyne, along with Hank Pym and Janet Van Dyne, explore the Quantum Realm, where they interact with strange creatures and embark on an adventure that goes beyond the limits of what they thought was possible.

When Scott Lang and Hope van Dyne, along with Hope's parents, Hank Pym and Janet van Dyne, and Scott's daughter, Cassie, are accidentally sent to the Quantum Realm, they soon find themselves exploring the Realm, interacting with strange new creatures—wellsnoah-89768

During her days of entrapment in the Quantum Realm, Janet van Dyne encounters Kang. In the present day after the Battle of Earth, Scott Lang has become a successful memoirist and has been living happily with his girlfriend Hope van Dyne. Lang's now teenage daughter Cassie has become an activist whose activities result in Lang bailing her out of jail.

While visiting Hope's parents, Hank Pym and Janet, Cassie reveals that she has been working on a device that can make contact with the Quantum Realm. Upon learning of this, Janet panics and tries to shut off the device, but the message is received, resulting in a portal that opens and pulls the five of them into the Quantum Realm. Scott and Cassie are found by natives who are rebelling against their ruler, while Hope, Janet, and Pym explore a sprawling city to get answers.

Hope, Janet and Hank meet with Lord Krylar, a former ally of Janet's, who reveals that things have changed since she left, and that he is now working for Kang, who is now the realm's ruler. After an intense showdown, the three are forced to flee and steal his ship. The Langs are told by the rebel's leader Jentorra that Janet's involvement with Kang is indirectly responsible for his uprising, they soon come under attack by Kang's forces, consisting of robotic subordinates led by M.O.D.O.K., who is revealed to be Darren Cross, having survived his apparent death at Lang's hands.

The Langs are taken to Kang, who demands that Scott help get his power core back or else he will kill Cassie. Lang is taken to the core's location and shrinks down. He is nearly drowned in a sea of variants of himself, but Hope arrives and helps him acquire the power core. However, Kang reneges on the deal, capturing Janet and destroying her ship with Hank on it.

After being rescued by his ants, who were also pulled into the Quantum Realm, rapidly evolved, and became hyper-intelligent, Pym helps Lang and Hope as they make their way to Kang. Still imprisoned, Cassie rescues Jentorra and they commence an uprising against Kang and his army. During the fight, Cassie appeals to Cross' character, which convinces him to turn sides and fight Kang, though at the cost of his life.

Janet fixes the power core as she, Pym, Hope and Cassie jump through a portal home, but Kang attacks Scott, nearly beating him into submission. Hope returns and, together with Scott, destroy the power core with a combination of Pym Particles and knock Kang into it, causing him to be pulled into oblivion. Cassie reopens the portal on her end for Scott and Hope to return home.

Now back home, Lang continues his happy life and orchestrates a fake surprise birthday for Cassie due to him missing so many. Lang briefly reflects on Kang's words and worries that he may not be dead or the door might now be open to the ones who exiled him, but decides not to dwell on that for now and focus on his extended family.

In a mid-credits scene: numerous variants of Kang are concerned by Earth 616's increasing interest in the multiverse and plan their uprising.

In a post-credits scene: Loki and Mobius M. Mobius spot another Kang variant, named Victor Timely, on Earth in the 1900s.