In the climax of the movie, Scott Lang (Paul Rudd), also known as Ant-Man, confronts the villainous Darren Cross (Corey Stoll), who has developed a weaponized version of the Ant-Man technology called the Yellowjacket. Scott, with the help of Hank Pym (Michael Douglas) and his daughter Hope van Dyne (Evangeline Lilly), must stop Cross from selling the dangerous technology to the highest bidder.
In the resolution, a final showdown takes place between Scott and Cross, with Scott using his shrinking abilities and the help of a swarm of ants to defeat Cross and destroy the Yellowjacket suit. However, during the fight, Cross is exposed to the Pym Particles and begins to suffer from cellular instability, turning him into a threat.
To prevent Cross from causing harm, Scott has to shrink to a subatomic level and enter the Quantum Realm, a dimension where time and space become irrelevant. With the help of a specialized suit, Scott successfully shrinks down, defeats Cross in the Quantum Realm, and manages to return to normal size.
The film ends with Scott reuniting with his daughter, Cassie (Abby Ryder Fortson), and making amends with his ex-wife, Maggie (Judy Greer), and her new husband, Paxton (Bobby Cannavale). Hank and Hope also reconcile, and Hope discovers a new Wasp suit that suggests she will take on the mantle of the Wasp in future adventures.
In summary, “Ant-Man” concludes with Scott Lang successfully thwarting Darren Cross’s plans, exploring the Quantum Realm, and setting the stage for future adventures involving the Pym Particle technology and the legacy of the Ant-Man and Wasp.