You must rescue three explorers, who are trapped at the bottom of deep caverns, by travelling down each cavern in your hot air balloon. Each cavern takes up several screens.
Controls are left, right and boost, which temporarily increases your height. As you are continually falling - perhaps the balloon has sprung a leak - to travel horizontally it is necessary to use frequent short bursts of boost, but it is very difficult to maintain any desired height. There's no joystick option.
The score depends on the height of the balloon in the cavern, which isn't a good measure of progress on many of the screens. The caverns are difficult to negotiate due to ragged edges and stalagtites, stalagmites and moving objects which all must be avoided. You can choose which of the three caverns to attempt; supposedly, easy, medium and hard, but I found them all extremely difficult. You also have the option of loading two other sets of caverns, but on my copy the program crashed while in the middle of loading them.
When selecting options, or starting, the key response is very slow, due to irritating tunes being played which can't be switched off.
Although the task is to avoid the alien in a confined space an incredible degree of coordination and timing is required. This is a simple idea made very difficult and would seem to guarantee hours of frustration.