On a return voyage from the African continent, the cruise ship Cradle mysteriously capsizes in the middle of the ocean. Inverted, torn apart, and sinking, the road to escape is now a hellish one as a result of the ship's twisted environment. Making matters worse for the remaining survivors, some passengers have turned mindlessly rabid and have begun attacking one another, spreading a contagious frenzy with each assault.
Student Makoto Ayukawa awakens in the midst of this situation. Fortunately, she is rescued by fellow classmate Yuuya Takigawa and the pair manages to regroup with more of their classmates. However, they remain unaware of a crucial factor essential to survival: choosing trustworthy allies, for the undead are not the only menace aboard the ship. With time being of the essence, the group begins their tumultuous ascent to escape the cradle of monsters alive.