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The Wii U version of Super Smash Bros. Fusion contains a story mode dubbed Adventure Mode: Hero's Fusion. The story features many of the villains from Nintendo franchises teaming up in order to destroy the heroes once and for all, only to then be confronted by a greater enemy.
As the fighters return to their worlds after saving the World of Trophies in Brawl, the villains go back to their evil ways: kidnapping princesses or conquering the world. With their own universes having succumbed to the villains and their new plots, the heroes come together to defeat each other's villains for a change of pace.
Bowser and Ganondorf are the only bosses that both must be fought and are later playable. Every other playable fighter that appears as a boss is optional.