Plot

Scott Lang and Hope Van Dyne are dragged into the Quantum Realm, along with Hope's parents and Scott's daughter Cassie. Together they must find a way to escape, but what secrets is Hope's mother hiding? And who is the mysterious Kang?

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

In this thrilling new adventure, Scott is living life to the fullest as he tries to juggle between his normal life and superhero life as the titular Ant-Man. When his now-teenage daughter, Cassie, builds a mysterious device in the basement, the device malfunctions as it sends both her, Scott, Hope and her parents down into the Quantum Realm. From there, the two families all try their best to survive whilst also encountering a lot of the realm's mysterious inhabitants and settings. However, they are forced to come into blows with a ruthless and tyrannical conqueror who threatens the safety of the multiverse.—Affan Jamsari

Some time after the Blip, Scott Lang returns to living a full life knowing that he helped save the world. But now, Scott along with his family are accidentally transported down to the Quantum Realm. Down there, they discover a world of unimaginable people that inhabit the Realm. But they soon discover that a rebellion is at war with the warmongering Kang the Conqueror who intends to conquer the Multiverse. Now Scott must do everything he can to return home with his family and stop Kang from causing mass chaos.—Blazer346

During her days of entrapment in the Quantum Realm, Janet van Dyne (Michelle Pfeiffer) (Pym's wife, Hope's mother, and the original Wasp, who was lost in the Quantum Realm for 30 years) encounters an exiled traveler named Kang (Jonathan Majors). Kang is a "time-traveling, Multiversal adversary" trapped in the Quantum Realm who needs Pym Particles to get his ship and a device online that would allow him to go anywhere and when in time.

In the present day, after the Avengers' battle against Thanos. Scott Lang (Paul Rudd) (An Avenger and former petty criminal with a suit that allows him to shrink or grow in scale while increasing in strength) has become a successful Memoirist and has been living happily with his girlfriend, Hope van Dyne (Evangeline Lilly) (The daughter of Hank Pym and Janet van Dyne who is handed down a similar suit and the Wasp mantle from her mother). Hope has taken back her father's company and is using the Pym Particle for the betterment of humanity. Scott's now-teenage daughter Cassie (Kathryn Newton) has become a political activist in helping people displaced by the Blip, resulting in her having a strained relationship with her father.

While visiting Hope's parents, Hank Pym (Michael Douglas) (A former S.H.I.E.L.D. agent, entomologist, and physicist who became the original Ant-Man after creating the suit) and Janet, Cassie reveals that she has been working on a device that can establish contact with the Quantum Realm. Upon learning of this, Janet panics and forcefully shuts off the device, but the message is received, resulting in a portal that opens and sucks the five of them into the Quantum Realm. Hank and Cassie's ants, who by now are highly evolved are also sucked in. Scott and Cassie (who are separated from the others) 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 Pym meet with Lord Krylar (Bill Murray) (The governor of the lavish Axia community in the Quantum Realm), a former ally of Janet's, who reveals that things have changed since she left, and that he is now working for Kang, the Quantum Realm's new ruler. The three are forced to flee and steal Krylar's ship. The Langs, meanwhile, are told by rebel leader Jentorra (Katy O'Brian) (The leader of the Freedom Fighters rebelling against Kang's oppression of the communities in the Quantum Realm) that Janet's involvement with Kang is indirectly responsible for his rise to power.

The rebels soon come under attack by Kang's forces led by M.O.D.O.K. (Mechanized Organism designed only for killing), who is revealed to be Darren Cross (Corey Stoll) (Pym's former protege who was shrunken to subatomic size in the Quantum Realm during the events of Ant-Man (2015) and became a mutated, Cybernetically enhanced individual with an oversized head known as M.O.D.O.K.), having survived his apparent death at the hands of Scott.

Aboard Krylar's ship, Janet confesses to Hope and Pym why she wanted nothing to do with the Quantum Realm again; Kang claimed that he and Janet could both escape from the Quantum Realm if she helped him rebuild his Multiversal power core. After they managed to repair it, Janet saw a vision of Kang conquering and destroying entire time-lines. Kang revealed he was exiled by his variants out of fear, which drove Janet to turn against him. Outmatched, Janet used her Pym Particles to enlarge the power core beyond use. Kang, having regained his powers, eventually conquered the Quantum Realm afterward.

The Langs are taken to Kang, who demands that Scott helps get his power core back or else he will kill Cassie. Scott is then taken to the core's location and shrinks down. He is nearly drowned in a sea of variants of himself (as Scott approaches the core, he is in the storm of probability where every possible version of himself exists simultaneously), 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 Scott and Hope as they make their way to Kang.

Kang reveals his plan to use the power core to transport his entire army to the multi-verse to eliminate his other variants and to take complete control of the time-lines. But the uprising brings the entire Quantum realm to fight against Kang and his forces are near defeat. Kang is forced to enter the battle himself, allowing Janet to escape captivity and pursue the power core. Kang forces the rebels to run for cover and the revolution is almost defeated. This is when Hank's ants enter the battle and turn the tide in the rebels favor. During the fight, Cassie convinces Cross to switch sides and fight Kang, though he sacrifices his life. The ants are technologically superior to Kang and are able to easily overrun him.

Janet fixes the power core as she, Pym, Hope, and Cassie jump through a portal home, but Kang attacks Scott. Before he can beat Scott into submission, Hope returns, and she and Scott throw Kang and Pym Particles into the power core, destroying them both. Cassie reopens the portal for Scott and Hope to return home. As Scott happily resumes his life, he begins to rethink what he was told about Kang's death being the start of something terrible happening but brushes it off.

In a mid-credits scene: numerous variants of Kang, led by Immortus, commiserate Kang's death and plan their Multiversal uprising.

In a post-credits scene: Loki and Mobius M. Mobius encounter another Kang variant, Victor Timely, on Earth in 1901.