The evil Jaku Empire from the 5th dimension has decided to invade the 3rd dimension, starting with Earth. Eldran piloting the mecha Raijin-Oh stops a missile loaded with Akudamas, but gets defeated in the process. It crash lands into Hinobori's elementary school. With the last of his powers, Eldran saves the ones present in the school at the moment: class 3 of the 5th grade. He entrust them with Raijin-Oh and the duties to protect the Earth, while Eldran goes into hibernation to recover his powers. Now dubbed as the "Earth Defense Class", the children of class 3 of the 5th grade need to fend off an invasion from the 5th dimension.