Crewman Nagato and Consulting Engineer Daiki are onboard the Special Purpose Submarine Asahi for an investigation. Daiki is the son of the Asahi's captain, but they're not actually related by blood. Nagato happens to witness the secret activities that take place in the Captain's cabin every night. Not only that, the Weapons Officer also comes to the Captain's cabin---. What really is the daily life of these men on the tide!? And what is the Captain's surprising tendency!? It's the beginning of a full-scale Yaoi fleet.