Throughout the game you'll be saving your slimy friends from evil. Most of them have been kidnapped and you'll have to fight a giant robot to get him/her/it back. Saving these little guys is important because certain parts of your town are blocked off until a certain number of slimes are saved.
These battles are the most unique thing about the game and can be really challenging, but at the same time its extremely fun and doesn't really get old. The plot is charming but its not really that important, however chances are you will grow to like this blue little slimy protagonist by the end.