In JetPac, you control an intrepid spaceman, who goes planet to planet collecting treasures while avoiding and shooting up the irate aliens. He starts off by assembling his rocket, and refueling it, and then moving on to the next planet. The planets only differ in the design of the aliens, and the pattern of their motion. On each planet, the spaceman collects treasures and refuels the rocket.
There are some very clever bits of gameplay which are fascinating. One, the screen wraps around. Going off the side of the screen just brings you around to the other. Of course, this applies to the spaceman, the aliens and even the shots fired by the spaceman. Second, the animation of aliens exploding (or the Spaceman himself, in that unfortunate eventuality) is very well done. Third, the graphics and colouring is very crisp and well defined. The movement of both the spaceman and the aliens is smooth and responsive. The fuel canisters and treasures slowly and smoothly waft down from the sky.
There are about 10 different alien designs, with about just as many movement patterns. There are 4 spaceship designs, and every five planets, a new spaceship has to be reassembled. So essentially, there are about 20 or so unique levels. The 21st level is identical to the first.
This game has provided endless hours of fun, and I remain continually amazed at what the developers have managed to wring out from a measly 48KB RAM with an 8-bit processor.