Cassie Lang held her seventh birthday party at her home when Scott Lang arrived after being released from San Quentin State Prison. Paxton quickly confronted him and made it clear that he was not welcome as Lang had failed to pay child-support. They continued to argue, while Lang gave his daughter her birthday present. Eventually, Maggie arrived, intervened and took Lang outside to discuss what he needed to do to see his daughter after Lang had accused him of being an ass-hat. Paxton stood with Cassie as Lang left the party and drove away.
While Scott Lang was taken away by Jim Paxton and Gale, Darren Cross was able to get inside Maggie Lang's house and take Cassie Lang hostage. Hearing of this prompted Lang to escape and battle Cross once again. With an army of ants as reinforcements, Lang engaged Cross on Cassie's toy train set. During the duel, Lang and Cross threw sections of the Thomas the Tank Engine toy at each other. As ants attacked Cross, Lang was able to knock Cross onto the train tracks, causing him to get run over by Thomas the Tank Engine. Cross retaliated by throwing the toy at Lang, but Lang knocked the train aside and onto the windowsill.
Meanwhile, Luis, Dave and Kurt arrived in their van outside to help Lang, but seeing the house surrounded by police, they were forced to retreat to avoid arrest.
Back inside, Lang tried to use Pym Particles Disks to enlarge Cross but he deflected them, causing an ant living there and the toy train on the windowsill to grow to gigantic size. Paxton attempted to rescue Cassie, but the Giant Pet Ant trampled him on the stairs. Eventually, Paxton made it to Cassie and tried to protect her from Cross. Lang continued fighting Cross and decided to go subatomic to sabotage the Yellowjacket suit from the inside, causing Cross to shrink fatally into the Quantum Realm, finally killing him. Lang succeeded, but also became trapped in the Quantum Realm; however, he had an enlarging Pym Particles Disk remaining and used it to reverse the suit's mechanism, successfully returning to normal size.
After the Battle of Earth
After the Avengers and their allies defeated Thanos and his army, Lang and Hope van Dyne moved in the house hoping to start their lives again. They watched fireworks with Cassie celebrating the people returning from the Snap.