NOTE: This spoiler was submitted by Jeremy

The film opens with a flashback in the Quantum Realm. Janet Van Dyne (Michelle Pfeiffer) comes across two creatures that attack her. She kills one but is pounced on by the other. Before it can finish her, someone blasts its head off. Janet turns and sees another man (Jonathan Majors) appears to have crash-landed in the Realm with her.

In the present day, Scott Lang (Paul Rudd) has been enjoying his newfound celebrity status after helping the Avengers save the world from Thanos (although at least one person mistakes him for Spider-Man). Meanwhile, Hope Van Dyne (Evangeline Lilly) has taken over her father Hank’s (Michael Douglas) company and has been using Pym Particles for expanding her research and using it for global benefits. Scott has also written a best-selling book about his experiences. However, his absence from being stuck in the Quantum Realm has also kept him from his daughter Cassie (Kathryn Newton).

Scott gets a call from jail and meets Hope there. Cassie was arrested for being at a protest. She has also been tinkering in science ever since The Blip, even beginning to work with Hope and Hank (whom Cassie refers to as “Grandpa Hank”). Scott and Cassie join the Pyms at home for dinner, where Hank accidentally lets it slip that Cassie has been arrested more than once. She draws attention to the work they have been doing, including Hank having an ant farm with his smart ants, while Cassie has worked on a device that sends signals to the Quantum Realm, and signals can be received in the outside world too. This causes Janet to panic, and she shuts the device off. Before she can explain herself, the device reactivates and starts pulling everything and everyone inside.

Scott jumps in after Cassie and uses his Ant-Man suit to protect her as they fall into the Quantum Realm. Hope uses her Wasp suit to save her parents. The Pyms gather as Janet tries to keep them from being spotted. They are found by inhabitants that Janet confronts. After a quick duel with one of the inhabitants, Janet and the creature laugh it off like old friends. She is then able to get her family a ride atop a large flying creature.

The Langs are found by other Quantum Realm inhabitants, varying from humanoid to many alien-like beings. Scott and Cassie must drink a red fluid called ooze that comes from one of the creatures, Veb (David Dastmalchian), which allows them to understand the natives. They are met by another native, Quaz (William Jackson Harper), who can read minds, as well as Jentorra (Katy O’Brian), a rebel warrior who suspects that the arrival of the Langs and Pyms will draw attention from someone they call “The Conqueror”. When Scott mentions Janet’s name, Jentorra becomes more suspicious.

Janet leads Hank and Hope to a bar full of other Quantum natives. They meet with a man called Lord Krylar (Bill Murray), whom Janet befriended (and is implied to have had a relationship with) during her time trapped down there. While Janet tries to get Krylar to help, he tells her that he is working for Kang, the Conqueror referred to from earlier. Krylar also alludes to Kang’s greatest weapon, a Mechanized Organism Designed Only for Killing (M.O.D.O.K.). Kang’s henchmen show up to take in the Pyms, but they fight their way out, and Hank uses one of his growing discs to turn a squid creature gigantic to grab Krylar and throw him around. The family then steals his ship and escapes.

The rebels soon come under attack by Kang’s forces, before M.O.D.O.K. makes his appearance and begins to vaporize some of the freedom fighters. In the chaos, Cassie reveals she has been wearing her own super suit to help her shrink and grow. M.O.D.O.K. personally confronts Scott and Cassie and reveals himself as Darren Cross (Corey Stoll), who survived his fight with Scott when he went subatomic and destroyed his Yellowjacket suit, which deformed Cross’s body and left him as a giant head with tiny limbs and a stretched out face. He was also the one who sent the signal to Cassie that she responded to. M.O.D.O.K. then brings the two to Kang.

On their way out, Janet comes clean to Hank and Hope about her time down there. When she had met Kang, he gave her the impression that he was another lost traveler, and that she could help him power his ship to get them out of the Quantum Realm, promising her he will return her to Hope. After Janet found his ship’s core, she learned that his ship was powered by his thoughts. When she touched the core, she caught a glimpse into Kang’s mind, and she witnessed all the atrocities he committed, from wiping out entire planets and destroying timelines. Kang attempted to appeal to Janet by saying he can make it so that she never missed Hope’s childhood, though she knew it would mean other worlds would suffer because of him. Seeing that he was a monster, Janet used her Wasp suit to steal the core and blow it up to a massive size to prevent herself or Kang from escaping.

Scott and Cassie meet Kang, who boasts about having killed other Avengers across other timelines. He demands Scott’s help in getting his core back, threatening to kill Cassie and force Scott to relive the moment for eternity if he doesn’t help. Scott reluctantly agrees but makes it clear he will come for Kang if he hurts Cassie. Scott is dropped down into the core’s location to retrieve it, but he gets caught in what M.O.D.O.K. calls a probability storm, causing hundreds of multiple Scotts to pop up. The Pyms fly by the location when Hope gets a read on Scott. As she flies down to help, she gets surrounded by her own probabilities. Scott’s probabilities manage to help him since their entire shared goal is saving Cassie. Hope helps Scott, and they shrink the core down together.

Hope and Janet try to help Scott now that he has the core, but Kang shows up to take it from him, going back on their deal to let Cassie go. He also takes Janet with him and has M.O.D.O.K. destroy the ship with Hank in it (though he survives). When Janet calls Kang a monster, he explains that he is “the exiled one” among his many variants, and he believes they are responsible for other horrors across the multiverse. He thinks that his escaping the Quantum Realm will allow him to take control over an event that is coming for humanity.

The ants from Hank’s ant farm arrive, after apparently having gone through time displacement that has helped them grow more advanced over less time than what the humans have gone through. They help Hank get Scott and Hope to head toward Kang’s stronghold. Meanwhile, Cassie uses her suit to shrink into a prison and free Jentorra and other rebels to charge toward Kang’s tower. Just as Kang tries to make a big villain speech, Cassie hijacks it and tells everyone to head to the tower to fight Kang. The other rebels are freed and fight off Kang’s goons. M.O.D.O.K. goes for Cassie, but she turns giant and punches him down. Before leaving, she convinces him that he doesn’t have to keep being evil.

Scott grows gigantic as he angrily runs toward Kang for reneging on their deal. He reunites with Cassie and is proud to see she is giant too. Hank leads the ants, plus a giant mecha ant toward the tower. Kang leads his own attack against the rebels, but M.O.D.O.K. goes in for his own attack now that he has switched sides. He chooses to die as Darren, and the heroes stand by his side as he dies.

The heroes make their way into the tower, where Janet has repaired Kang’s core and has opened a portal to return home. Scott pushes Cassie through just before Kang attacks him. They get into a brutal fight, with Kang nearly beating Scott to a pulp. Kang attempts to jump through the portal, but Hope jumps back to help Scott fight. They overpower Kang and destroy the core by using multiple growing/shrinking discs on it. Hope pushes Kang toward the core, which pulls him into a portal to oblivion. Cassie manages to reopen the portal to help Scott and Hope get home, while the Quantum natives cheer for their victory.

Scott returns to his life and family, although he worries about what Kang said regarding something terrible coming. He brushes it off as nothing and joins the family to celebrate a fake birthday for Cassie since he missed some during The Blip.

Mid-Credits Scene: A council of Kang variants discuss the death of “The Exiled One” and prepare to send each of themselves out into the multiverse as they prepare for a major war.

Post-Credits Scene: Another variant of Kang, Victor Timely, is seen teaching a class in the early 19th century. Sitting among the class is none other than Loki (Tom Hiddleston) and Agent Mobius (Owen Wilson). Loki tells Mobius that this man is going to be a very dangerous threat.

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After helping the Avengers save the universe, Scott Lang/Ant-Man has been enjoying his newfound celebrity status, while Hope Van Dyne/The Wasp has been advancing her scientific endeavors. Scott just regrets being stuck in the Quantum Realm for years and missing out on his daughter Cassie's life. She has since taken an interest in science, working with Hope's parents Hank and Janet, but a device that Cassie created to send a signal to the Quantum Realm causes the five of them to get pulled down there.

In the Quantum Realm, Scott and Cassie meet some of the natives of the area, who are rebels fighting against a villain called Kang the Conqueror. During her time down there, Janet befriended Kang, believing he was another lost traveler who needed help getting home and would have helped Janet reunite with Hope and Hank. However, she learned that he was responsible for countless atrocities across the multiverse, destroying worlds and timelines as he saw fit. Janet blew up Kang's ship core to prevent either of them from getting out. Kang sends M.O.D.O.K. (who is really Darren Cross/Yellowjacket that became deformed when Scott destroyed his suit and had him sent to the Quantum Realm) to retrieve Scott and Cassie, then forces Scott to retrieve the core or he will kill Cassie.

Hope helps Scott retrieve the core, but Kang reneges on the deal and doesn't let Cassie go. Cassie escapes using her own super suit and frees the rebels to attack Kang's tower. Hank also uses some super ants in the fight, and Janet opens a portal using Kang's core to get them home. Scott fights Kang to prevent him from escaping, and with Hope's help, they destroy the core and cause Kang to get pulled into oblivion before Cassie brings them home.

Scott returns to his life but has a lingering worry that the threat of Kang is only beginning. Sure enough, a council of Kang variants catches wind of the death of one of their own and gets ready to send each of themselves into the multiverse for an oncoming war.