From the game manual:
Become a Cosmo Cop!
There is only one thing standing between the world and total disaster - you! Take control of either Leo or Jenny and do battle with the Babools, a lifeform dedicated to the pursuit of pure evil. Select the World and Stage you want to play and clear them one by one to progress through the game. The order in which you play the Worlds will vary depending on whether you choose to play as Leo or Jenny.
Flick, run and slingshot your character from rock to rock, holding the Nintendo DS horizontally or vertically, to avoid enemies and reach the Goal before the timer runs out. Collect gems, power-ups and keys to aid your quest and unlock extra stages.