Climate warming intensifies, country Z's environment deteriorates, and world resources begin to be scarce. In 2018, country z and country m began a cold war. In 2020, country z and country e began to establish diplomatic relations, and country m and country r began to unite. In 2023, country m dropped poison gas bombs on country z without notice, and the zombie virus broke out. In 2029, country m was unable to save the situation, zombies invaded the earth, and humans lingered. In 2035, people call it the era of despair.