1989 - Hank Pym (Michael Douglas) enters a SHIELD facility to confront Howard Stark (John Slattery), Peggy Carter (Hayley Atwell), and Mitchell Carson (Martin Donovan) over trying to replicate a serum of his that he refers to as the "Pym Particle." If something like this fell into the wrong hands, it could spell chaos for the world. Carson makes a comment about Hank not being as ferocious in saving his wife Janet. Hank responds by slamming Carson's face down on the table. Hank resigns.
In the present, Scott Lang (Paul Rudd) is getting punched in the face in prison. He hits the other inmate back, and then they shake hands, as this is part of a goodbye ritual. Scott leaves prison and is picked up by his friend Luis (Michael Pena). Luis hooks Scott up with a place to stay with his roommates Dave (T.I.) and Kurt (David Dastmalchian), as well as a job at Baskin Robbins. Unfortunately, Scott is let go due to his criminal history, despite the manager thinking his robbery crimes are cool. Scott returns home to lament the loss of his job. Luis tells him that he got a tip about a job that involves stealing something, but Scott says that life is behind him.
At Pym Technologies, Hank returns after many years to the surprise of those now working there. He reunites with his daughter Hope Van Dyne (Evangeline Lilly), as well as his former protege Darren Cross (Corey Stoll), who now runs the place. Hope has been estranged from her father since her mother's death. Cross leads Hank and company into a room to unveil his latest project, a weaponized suit based off Hank's old Ant-Man project - the Yellowjacket. With it, the user can shrink to a very small size and defeat enemies using the strength applied to the suit's capabilities, and Cross hopes to create an army of Yellowjackets. One man named Frank (Joe Chrest) doesn't like the presentation, citing the project as expensive and dangerous. Cross later finds Frank in the bathroom and vaporizes him into a blob of goo, which he wipes up and flushed down the toilet.
Scott makes a surprise appearance at a birthday party for his daughter Cassie (Abby Ryder Fortson). Her mother's fiancé Paxton (Bobby Cannavale) is a cop and doesn't approve of an ex-con showing up. Scott's ex-wife Maggie (Judy Greer) is even less thrilled to see Scott. Cassie, however, is very happy to see her dad, and she loves the ugly little doll that he gives her as a gift. Scott goes back to the apartment and asks Luis what this tip was that he heard about. Luis learned through several sources that there is a home that contains a safe with something worth taking. Scott says he's in.
The guys drive Scott to the home and stay in his ear to help him out. Scott hops the fence and claims the windows, killing the alarm for 20 minutes. He finds a door with a thumbprint scanner and uses several nearby items to get a fingerprint to open. Behind the door is a room with the safe, but it's made of a particularly strong metal. Scott cools the metal to cause it to break open. Inside the safe is no money, but some blueprints and the original Ant-Man suit. Scott reluctantly takes it. A little ant with a camera crawls nearby and records Scott.
At home, Scott tried the suit on and steps into the bathtub. He presses the button on the right glove, and he shrinks down to a tiny size. Already in shock, Scott then hears Hank's voice speaking to him through the mask. Luis then enters to take a shower, splashing Scott with water as he turns it on. Scott gets out of the tub and falls through a hole in the apartment below, where a loud party is going on. Scott narrowly avoids getting stepped on before falling through the vent where he encounters a mouse that is significantly larger than him. He causes it to run into a trap and is launched out the window where he returns to his normal size after landing on a cab.
Scott decides to sneak back into the house and return the suit. Unfortunately, the cops are already outside and they arrest Scott. As he sits in his cell, Paxton tells Scott that Maggie and Cassie will be disappointed to learn he got locked up again. Scott is then told that his lawyer is there to see him. It's Hank in the interrogation room, saying he was the one that had been speaking to Scott and that he set it up from the beginning to get Luis's friends to tip him off about the job. His ants crawl over the camera to obscure the conversation. Hank criticizes Scott's choices to go to crime when things look down.
Several controlled ants bring Scott the suit in its tiny form, then turn it big so Scott can quickly slip into it and escape. The police go on a manhunt for him when they see the empty cell, though Scott is already flying out on the back of a winged ant. As they fly, Scott feels dizzy and he falls off the ant's back.
Scott wakes up in the bedroom of the house he broke into. Hope is in the room, unhappy to see him. Scott then finds Hank again, who explains to him why he needs Scott. He knew of his skills when Scott pulled off a heist from a supposedly tightly secure location, and he knew he could steal the suit. Hank explains why using the Ant-Man suit would be dangerous in the wrong hands. Scott suggests they call the Avengers for help, but Hank already refused to work with Howard and is not about to let Tony get his hands on this technology. Scott thinks this is risky, as does Hope, who thinks of anyone should use the suit, it should be her, but Hank refuses. Hank persuades Scott to help so that he can be a hero to Cassie. Scott says his days of breaking into places and stealing shit are over. Hank says he needs Scott to break into a place and steal some shit.
We see Cross performing shrinking experiments with baby goats. The first goat gets vaporized into goo. The second one is placed in a glass container with a different serum, and it is shrunken without negative results.
Hank and Hope train Scott to properly use the suit. He needs to be fast, agile, and able to communicate with the ants the same way Hank does. Scott fails many times until he talks to Hope, who says to think of Cassie as motivation to get the ants under his control. With that, he gets the ants to spin a penny. Over time, Scott uses the suit properly and befriends the ants, naming a flying ant Antony.
Paxton receives a text informing him that Hank was posing as Scott's lawyer. Cassie asks Paxton if he got good news. Paxton says its news and that they will find her dad. Cassie says she hopes they don't catch him.
Scott is tasked to fly to a base to retrieve a particular item. Turns out nobody knew this was an Avengers base. Scott takes Antony and swoops down to encounter Sam Wilson/Falcon (Anthony Mackie). He spots Scott despite his small size and forces him to come clean. They fight but Scott gets the upper hand and disables Falcon's flight pack. He asks the others not to tell Cap about it.
Hank finally tells Hope what really happened to their mother. Enemy forces sent a missile toward the U.S., so Hank in his Ant-Man suit and Janet in her Wasp suit intercepted the missile but couldn't break into it and jam it. Janet compromised her regulator and went subatomic, shrinking her small enough to stop the missile, but causing her to keep shrinking into a reality from which she could not escape. Hope cries and forgives her father. Scott ruins the moment by commenting on how they are mending the relationship.
Later, Cross visits, so Hank shuts the door to keep Scott and Hope hidden. Cross invites Hank to a charity event the following evening. He also asks Hank why he pushed him away when he was mentoring Cross. Hank says he saw too much of himself in Cross. After he leaves, Hank decides they can make the steal at the charity event. Knowing there will be tight security, Scott says he can sneak in through the water system, and he enlists the help of Luis, Dave, and Kurt (to Hank and Hope's dismay).
The gang goes to the charity event. Hope gets in by attendance while the guys help Scott and the ants get in. Luis sneaks in as a worker, avoiding being compromised and lowering the water level. Paxton and his partner spot Hank and try to arrest him. Dave creates a diversion by stealing their car, allowing Hank to get in. After he gets back to the van, he accidentally honks the horn, which has the tune of "La Cucaracha", which Paxton recognizes from Scott using it earlier. They find Dave and Kurt and try to arrest them, but Kurt manages to take out the laser grid above the Yellowjacket suit. Scott makes it in, only to be found out by Cross as he is with Hank, Hope, Carson, and several HYDRA buyers. Cross attempts to kill Hank, but Scott breaks out using some discs Hank gave him that turn things large. Hank is shot in the chaos, but non-fatally. Cross makes off with his suit in the chopper. Scott runs after him while Hope stays with Hank.
Scott makes it to the chopper, where Cross tries to shoot him. Cross hits Antony mid-flight, to Scott's horror. He vows to make Cross regret it. Cross puts on the Yellowjacket suit and fires lasers at Scott, hitting the pilots. Scott sets off charges set around Pym Technologies and destroys the whole building. As Scott and Cross shrink, they fall out of the chopper in the briefcase fighting. They land in a family's pool. Scott causes a shrunken Cross to fly into a bug zapper. Paxton and his partner show up to arrest Scott.
On the ride back to jail, the cops hear of a disturbance from the Lang home. Scott knows Cross went after Cassie. Sure enough, he's standing in her room menacingly, holding her hostage while Maggie is outside helpless. Scott arrives and battles Cross. They fight all around the room, shrinking and growing. Scott throws a disc to make an ant and a Thomas The Tank Engine toy grow. Paxton runs to the room to protect Cassie. Cross aims his lasers at them as a tiny Scott tried to break into the Yellowjacket suit. The titanium is too strong for him to break into, so he realizes he has to go subatomic, despite Hank's warnings. Scott shrinks to a small enough size to break into Cross's particle pack, causing him to also go subatomic and disappear into oblivion.
Scott floats through subatomic space and hears Cassie's voice calling to him. He takes a disc and places it in the reactor, allowing him to return to normal size. He embraces Cassie.
Later on, Scott has dinner with Paxton, Maggie, and Cassie. As gratitude for his heroism, Paxton lets Scott off the hook and covers for him. They then watch a video of Cassie doing her first cartwheel.
Hank questions Scott as to how he got out of subatomic space and wonders if Janet could have gotten out too. He then catches Scott and Hope in the middle of a kiss. Scott plays it off but Hank knows what's up.
Scott rejoins his friends to hear Luis tell him about another tip for another job. Falcon is looking for someone with a special set of skills, and Luis knew just who to ask.
Mid-Credits Scene: Hank brings Hope down to his lab. There, he presents her with a prototype Wasp suit that he and Janet decided should go to Hope. Hope smiles and says, "It's about damn time."
After Credits Scene: Falcon joins Steve Rogers/Captain America (Chris Evans) in a dark room where they find Bucky Barnes (Sebastian Stan). Cap doesn't want to tell Tony about this. Falcon says he knows a guy.
The final text reads "ANT-MAN WILL RETURN".
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