Aliens invaded the earth and people were forced to take refuge in giant bunkers underground. S.U.M.1 is a loyal warrior who is sent to the ground to rescue the survivors and report on the situation every day. His mission was to guard the human self-defense zone from aliens, but this daily routine gradually became his nightmare, with frightening voices echoing in the woods; the base was frequently attacked every day; the power supply in the tower broke down; the repairman didn't show up. His boss never told him about the previous one. Loneliness and paranoia slowly eroded his reason. Are aliens really ambushing in the forest? Or has S.U.M.1 been tortured to insanity?