In 2125 a special unit SIS was created to defend against extraterrestrial life. It consists of five female members - Erie, Louis, Megu, Remy and Io. After investigating an unidentified spacecraft heading for Earth, Erie was attacked by a scientist Baikal who vanished 150 years ago. Io is captured and the rest of the crew, except for Eri, are forced to rescue Io and ride giant robot Guynarock to oppose the giant robot of Baikal.

[From vndbreview]