"Adventure Time: With Gravity Falls 2: When Heroes Unite!" is a crossover about when Finn and Jake meet Dipper and Mabel Pines again, but the Ice King returns and captures Princess Bubblegum and Mabel Pines. And they once again team up together to defeat him, but the Wax Figures return back to life and kidnap Grunkle Stan, Soos, Wendy, Marceline, Slime Princess, and Lumpy Space Princess. Leaving them to defeat both their enemies and return back to their normal worlds.
Finn and Jake once again find Princess Bubblegum's teleport device, but while teleporting the Ice King and their friends saw them leaving to Gravity Falls. But then the wax figures and Dipper and Mabel appeared. Then Finn and Jake have fun with Dipper and Mabel Pines until the Ice King and the Wax Figures capture all of their friends. Then they both team up to defeat them.