You're located on one of the Saturnian moons armed with guided missile cannon. The moon isn't vacant but inhabited by aggressive crabs. These creatures are descending from horizon near to you and than starts shooting. After some time they're returning back and than down again (unless you kill them of course). Your only defense is side-stepping and firing. There is no real level advance, only new hordes of Luna crabs and 5 lives in your possession.
B.L. Stryker (20699)