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Shrek Super Slam is first and foremost a party game, there are single player modes. You can choose from Story mode, Mega Challenge, Melee or Training.
Story mode is a series of created situations that lead up to a fight. There is a lead-in story and then you fight against the computer as one of the characters from the story. As I mentioned above, making it past a level in this mode can earn you additional costume changes for the characters.
Mega Challenge focuses on specific attacks, abilities or goals and challenges players to pull off a certain number of these within a time limit. This does provide a nice challenge, but also helps to train you in the various skills and moves used in the game.
Training is just that - you train at the various moves that you can execute with the different characters. This is done as a three stage tutorial - for moves of increasing levels of difficulty. This can be useful to help you work on those special moves and such, but doesn't offer the story/premise of the Mega Challenge mode.
The main mode in Shrek Super Slam is, of course, the Melee mode. This is the mode for which this game was truly designed. Get up to four gamers together, grab a multi-tap and some controllers, choose your characters and enjoy the free-for-all slam fest. The characters and the costumes that are unlocked are for use in this mode. If you've been playing Training mode, this is what you've been training for. Melee mode can be quite fun, given a suitable group of gamers. The gameplay can be quite fast when you really get going, and there are location-specific power-ups that can be utilized against your enemies, as well as some surprising environment interactivity.
While the Story mode is fun and the challenges are interesting, the real replay value is in playing against friends in the Melee mode. If you have a few friends that really like Shrek and like melee games, then this game is an obvious choice.