Target Earth is a space shooting game set in the future with Earth under attack by robots. The protagonist of the game is a pilot of a mech robot which can either walk on planet surfaces or fly in space. There are two types of levels in the game. Surface levels take place on planets or moons and allow the mech to run and jump to the end of the level while fighting enemies, as in a side scrolling game. Space levels have no gravity and allow the mech to move in any direction. The mech can be equipped with a large assortment of machine guns, lasers, and grenades.

It is the year 2201 and expanding technologies have given man the power to live anywhere in space. Amidst this backdrop, the story of Target Earth begins with a war between Earth (and its colonies) against a cyborg army returning from the farthest reaches of space. In the game's early stages, the nature or purpose of these cybernetic adversaries, named Chron, is unknown. However, it is later revealed that Chron are the survivors of a failed space expedition (Outer Space Expeditionary Party) sent by Earth a hundred years earlier.

The Earth Defense League fights to defend Earth. The heart of the defensive movement is the Assault Suit - a twelve-foot tall armored battle machine with awesome fighting capabilities. You are Rex, an Assault Suit Wing commander and master at Assault Suit combat. The battle begins on Ganymede but shifts to battles in space, on the Earth, and inside enemy outposts.