The event that sets the rest of the movie in motion is Scott Lang's (Paul Rudd) bizarre vision of the quantum realm, which leads to him witnessing a younger Janet van Dyne (Michelle Pfeiffer) playing hide-and-seek with Hope as a child.
It's later revealed that the vision was a "quantum entanglement" event caused by Lang's first trip to the quantum realm in the previous movie, and was triggered by Hank Pym (Michael Douglas) and Hope (Evangeline Lilly) powering on the quantum tunnel.
Janet was able to plant a message in Scott's head...because reasons basically. Though the film makes a half-baked effort to explain Janet harnessing the power of the quantum realm, ultimately it's just a ton of nonsense to get the plot moving, and thankfully it isn't lingered on for too long.
Stay at home dad who spends as much time teaching his kids the merits of Martin Scorsese as possible (against the missus' wishes).
General video game, TV and film nut. Occasional sports fan. Full time loon.