A whale summons you into the ocean. By the end, you're fighting a militarized madman in a lost civilization's last defense. Submarine 707 has no time to explain itself, no interest in breathing room, it throws you into the deep and expects you to keep up.

A mysterious object is attacking and destroying every ship and submarine it meets. Submarine 707 is sent out to track it down, and during that search a whale summons the crew and leads them toward the world of Mu. Along the way they run into the mysterious object and learn that it is Commander Red Silver, who has attacked Mu in order to seize its magma sources. Submarine 707 goes into battle against Red Silver to save Mu, and the rest of the world along with it. The story is a single-episode sci-fi adventure OVA.