Mario versus Wario is a game for the Wii. It is a side-scrolling RPG.
Mario is in his house when Toad runs up. He says to come with him. Toad jumps into a pipe, with Mario close behind. Toad explains, after they exit the pipe, that Wario is trying to take over the Mushroom Kingdom. He says he doesn't know why Wario would do this, but he does know that Mario has to go to Wario's castle ASAP. Mario runs off and the game begins.
Mario goes to Wario's castle. He defeats him. Wario has only 450 HP. When defeated, Wario will fall over. His eyes glow red and a black wisp of smoke exits his body. He wakes up, asking where he is. Mario, who saw the smoke leave Wario, looks angrily in the direction it went. Suddenly, a giant black ghost enters the screen. Mario and Wario are scared. You enter battle and team up with Wario to defeat the evil ghost. It has 1000 HP. When defeated, it says it is an evil spirit that was trapped in a chest until a greedy Wario opened it. It possessed his body, but didn't count on Mario being able to defeat it. It then explodes, and Mario and Wario go to the Mushroom Kingdom. They are seen in the credits as a festival is taking place.
End of spoilers