A mysterious force is wreaking havoc by attacking and obliterating both ships and submarines. To identify the culprit behind these assaults, Submarine 707 is beckoned by a whale to trail it. The whale guides the team to Mu's domain, however, they come across the inscrutable entity during their journey. After investigating, they realize it is Commander Red Silver who conducted an assault on Mu to gain hold of its magma resources. With a calculated strike, Submarine 707 overthrows Red Silver and secures Mu and the planet as a whole from harm.