Yōhei Hayami commands the submarine 707 in Japan's Maritime Self-Defense Force, renowned as one of the finest submariners for his exceptional combat and leadership skills. He is married to Miyuki and they have a daughter, Ayu.

When the international Peace-Keeping Navy (PKN) forms to counter the terrorist group USR, Hayami leads an older-model Japanese submarine. At the PKN's inaugural meeting, Admiral Red's submarine UX attacks the fleet. Facing tactical disadvantages, the PKN vessels struggle to respond. Hayami deliberately maneuvers his submarine to intercept a torpedo targeting the flagship Apollo-Norm. This sacrificial act destroys the original 707 but saves the flagship, enabling Hayami and his crew to survive.

Six months later, Hayami returns to duty, taking command of a newly built submarine also designated 707. He assembles a mixed crew of veterans and recruits. His mission remains confronting Admiral Red and the USR, escalating into a prolonged personal rivalry. This conflict pits Hayami's seasoned expertise and dedication against Red's advanced technology and radical ideology seeking to end human exploitation of the oceans.

Later, investigating mysterious vessel attacks, Hayami and the 707 follow a whale to the underwater civilization of Mu. They encounter Commander Red Silver (linked to Admiral Red) assaulting Mu to seize its magma energy. Hayami engages in combat to protect Mu and neutralize this threat.

Hayami's leadership consistently inspires deep loyalty in his crew. His executive officer, Hayato Nango, rises in rank and eventually commands the submarine 717.