In the ancient world more than 6,000 years ago, the tragic war that tore the heaven and earth apart continued. Humans who had no power to fight back were forced into a desperate situation by various powerful races and were on the verge of extinction. At this time, Riku, the young leader of mankind, met the Mecha girl Shuvi by chance. Abandoned by his companions, Shuvi asked Riku to tell him the "heart of mankind" in order to repair the fault. The innocence and sincerity of the Mecha girl made Riku gradually let down his guard and open his heart to Shuvi, and the relationship between the two gradually heated up. In the end, the two led mankind to face the ultimate battle together, and the girl Shuvi would also face the cruel choice between lover and life. The end of the world is coming, and in 251 seconds, a deep confession is gambled.