In 1972, the Apollo 17 mission discovered a hypergate to Mars on the surface of the moon. Soon a war breaks out between Earth and Mars, and Martian soldiers begin to descend from the sky, riding steel giants, intent on exterminating humanity.
Humanity has successfully colonized all the planets in the Solar System, with intentions to continue into new frontiers. Individuals live stable lives in these colonies, including young high school students living in 4th Tokyo on Mars. Two of these students are Iris Shirazaki and Mizuki Sera. Iris is finishing her last year of high school while studying to be a test pilot while her friend Mizuki also works as a mechanic on the aircraft. Her own brother, Kaito, oversees Mizuki and Iris as their homeroom teacher and program director.
Spike Spiegel, a bounty Hunter with a dark past, lives on bounty to bounty, barely making it each time, or not making it at all. His partners Jet Black, Faye Valentine, Ed and Ein the dog all help him in his adventures from planet to planet.