Follow the screen around in a loop. Every lap you take of the arena makes the game harder. Enemies get stronger and faster, so you have to upgrade at the end of every lap to become stronger as well. I was going to add more variety as it goes on, until the screen was total chaos, but I knew I wouldn't have time.

There was a brief story involved, involving you being a surfer DUDE who had been eaten by a giant sea snake and you are in its guts. This was never going to be explained, assuming players would work it out from the game.