So Cassie Lang (Kathryn Newton) has turned on this device, which is sending a… signal? to the Quantum Realm. Far from being just the microscopic world, the Quantum Realm is actually a part of the Multiverse that is pretty hard to get to. You need magic or some Pym particles, and the first requires a lot of training and the second is in short supply. But this thing starts flashing, which is very likely Not Good.