Mankind's last defender is pitted against an endless wave of alien ships, hellbent on destroying the Earth using incredibly powerful explosives. You must outwit their evasive maneuvers and take them down before they carry out their nefarious scheme.You are given a strict time limit to hunt down each ship, and it gets shorter and shorter as the game progresses: take aim and fire before the countdown expires, or the enemy will plant its explosive charge in Earth's orbit. If ten of them succeed, the Earth is doomed.