In this game creatures attempt to get from the top of the screen to the bottom where they turn into Crabs which close in on you.
You need to defend yourself with a blaster that can fire nets or energy blasts. Nets trap the creatures and earn you double points, energy blasts kill them but sometimes only split them in which case you only get half points. if the screen turns yellow you must prepare to move your blaster as it moves up teh screen to the top (A-la' Tempest), once successfully at the top, the game will resume in a new play area.