Full story
After the victory over Thanos, Scott Lang has become a celebrity. He wrote a memoir, gets recognized on the street and lives happily with Hope van Dyne. His daughter, Cassie, now 18, is an activist who keeps getting in trouble with the police and pushes her father to do more for others. At a family dinner, Cassie shows off what she's been building with Hank Pym: a device that sends signals into the Quantum Realm. Janet panics and tries to shut it off, but it's too late. A portal opens and pulls all five of them in, along with the ants from Hank's lab.
Scott and Cassie land among rebels fighting the local tyrant. Hope, Hank and Janet end up in a city, where they meet Lord Krylar, Janet's old ally, who now works for the Quantum Realm's new ruler. The three steal his ship and flee. With the rebels, Scott and Cassie learn that Janet is directly tied to that dictator's rise. Soon his troops attack, led by M.O.D.O.K., who turns out to be Darren Cross, the villain Scott thought he killed in the first film. Scott and Cassie are captured.
On the ship, Janet finally tells the secret she kept for 30 years. She met a stranded man there who promised to get her out of the Quantum Realm if she helped him fix the power core of his time ship. When a mind link showed her that he wiped out entire timelines, Janet enlarged the core so it would never work and left him trapped. He is Kang the Conqueror, exiled by his own variants, and he ended up conquering the Quantum Realm.
Kang forces Scott to retrieve the core by threatening to kill Cassie. Inside, a probability storm creates hundreds of copies of Scott, and he only gets the core with Hope's help. Kang breaks the deal and captures Janet, and M.O.D.O.K. blows up the ship with Hank aboard. Hank survives, rescued by the ants, which have evolved over centuries of quantum time into a civilization. While Scott and Hope take on Kang, Cassie leads the Quantum Realm's people in revolt. She talks Cross into switching sides, and he dies sacrificing himself.
Janet fixes the core her own way and opens the portal home. Hank, Hope, Janet and Cassie go through, but Scott stays behind fighting Kang, who nearly beats him to death. Hope comes back to help, and together they throw Kang into the core, which is destroyed. Cassie reopens the portal and everyone gets home. Scott goes back to his easy life, but Kang's warning that his death would have consequences keeps nagging at him. In the mid-credits scene, a council of Kang variants gathers to plan the conquest of the multiverse. In the final scene, Loki and Mobius come across another variant, Victor Timely, at the 1893 Chicago World's Fair. All of it is left open.