A group of space pirates known as the Cosmo Gang is concocting an evil scheme, which involves smuggling rare fruits and using them for their own foul purposes. Two young mecha pilots, Riki and his sister Mami board a giant mecha called Bangai-O , in order to confront the pirates in battle and foil their villainous plans.
Bangai-O is an action-packed game that takes place in space. The player controls the titular mecha, Bangai-O , and shoots in eight directions. There are two firing modes: one for Riki and another for Mami. In this mode, Riki fires homing missiles, while Mami's shots bounce off walls. When enemies are defeated, they drop fruits that serve as power-ups. It is also possible to execute a special attack that consists of shots fired in all directions. Players can score points by performing successful combos or increasing the explosion count in the Dreamcast version.