Just as it seemed that the Xyolians had been expelled from the galaxy, a starprobe has reported underground activity on a small moon. Now it's up to you to penetrate the defences of their hidden base by controlling the probe.
As the game starts, your probe descends into the depths of a cave. Using keyboard control to hover, move left, right or fire either QuasiThermite bombs or the Superbeam, the probe must be guided down the maze of tunnels that scroll up the screen, blasting Xyolian defences all the way. Any fuel depots you see should be bombed to boost the probe's reserves. The same applies to ammunition dumps.
When the game ends you are given the total depth, reserve fuel and ammunition, and your deepest probe, although this information can be called any time during the game by the P, pause, key. You may play again in the same cave or in any of another eight different ones.
If you're looking for an entertaining program for your unexpanded TI this is an excellent choice. It's by far the best that I've seen for a long time that exploits the better qualities of this under-rated machine.