After a return journey from Africa gone wrong, The Cradle, a cruise ship, flips over without warning, leaving its passengers fighting for survival in a sinking and twisted environment. To make matters worse, some passengers have been taken over by an infectious frenzy, attacking their fellow survivors and making a desperate situation even worse. In the midst of this turmoil, Makoto Ayukawa wakes up and is rescued by a schoolmate, Yuuya Takigawa. Together with other students, they do their utmost to get out of the dire circumstances. However, they are unaware of the significance of selecting reliable allies as they make their way through the vessel, which is infested with both undead and dangerous human beings. As time runs out, the group struggles to leave the ship and to prevent their transformation into monsters.