Superhero partners Scott Lang (Paul Rudd) and Hope Van Dyne (Evangeline Lilly) return to continue their adventures as Ant-Man and The Wasp. When Scott's daughter Cassie Lang (Kathryn Newton) discovers a way to contact the Quantum Realm, she doesn't realize how dangerous it is. She, along with her father, Hope and Hope's parents Hank Pym (Michael Douglas) and Janet Van Dyne (Michelle Pfeiffer), accidentally find themselves falling down into the Quantum Realm.
Years earlier, when Janet was trapped there, she ran into Kang (Jonathan Majors) and after helping him, discovered his devious plan. Knowing how dangerous it is to be in the Quantum Realm, she tries to get them all out before they're stuck forever.