Unlike most action-role-playing games, which are set in medieval fantasy worlds, Darkspore is nominally science fiction set in outer space. Of course bad guys are trying to take over the universe, and you have to stop them. There is more than a bit of Pokémon in Darkspore in that there are 100 different hero characters you can unlock over the course of the game, each with its own special abilities.
From your personal menagerie you assemble teams of three creatures. You control only one hero at a time, while the other two are “off the board.” But if, say, your quick, slashing guy is low on health, you can swap out tag-team-style for, say, a big, slow tank character (or a long-range “magic” character or whatever else).