Mario vs. Sonic is an online multiplayer/co-op game for the Nintendo Wii, the fourth in a series of crossovers starring Mario and his former rival, Sonic the Hedgehog, besides the two Olympic games and Super Smash Bros. Brawl.
In the game, up to four players play as either Mario (the balanced type), Luigi (the power type), Peach (the magic type), or Yoshi (the speed type), online or locally, as they traverse through one of five campaigns, each based after one of five 2D Sonic games for the Genesis and Sega CD. Each campaign is divided into seperate zones based after the zones from their original games, and at the end of each zone do the players battle Sonic and/or any of his sixteen friends in boss battles. After defeating the group of bosses, Mario and friends head on to the next zone, until they reach the zone containing a fight with Super Sonic. From defeating Super Sonic, do they fight against Metal Sonic with just Luigi, Peach, and Yoshi, followed by a final battle against Eggman with Mario and Bowser (that's right, folks, Mario teaming up with Bowser to take down Eggman).
The game also comes with four-on-four online multiplayer, where one team plays as Mario, Luigi, Peach, and Yoshi, while the other team plays as four randomly-selected Sonic the Hedgehog characters. Regular modes include team deathmatch, capture the flag, bomb the base, and so on. The online multiplayer game also comes with the ability to pick between one of three fighting styles, including strength (high HP and defense), speed (higher combo movement), and magic (higher magic power), at the cost of another fighting style (sacrificing strength reduces HP; sacrificing speed reduces combo-attacks; sacrificing magic reduces magic power), as well as killstreak abilities. Get three kills without dying, and you activate your character's special move; collect five kills without dying and you get to summon either a Mario plane to drop bombs if you're on Mario's team, or the Tornado to do the same thing if you're on Sonic's team; collect seven kills without dying and you get to summon either koopas, goombas, bob-ombs, and other Mario enemies if you're on Mario's team, or badniks if you're playing on Sonic's team; collect nine kills without dying, and you can summon either an AI-controlled Bowser if you're on Mario's team, or Eggman if you're on Sonic's team.