After discovering the body of an old man in his home, Sans Undertale uncovers a legend about a summoned spirit who roams the town that he lives in. It is rarely seen, but whenever an eye is set upon this figure, they have exactly 10 days left to live. It is reported that after an encounter, victims endure hallucinations, brain tremors or in some cases; hypnotic states. Sans is on a journey to discover the truth about this spirit, whilst battling his own inner demons along the way. He recruits his ally, Thanos, to help him with his mission. Will they be able to stop the spirit before their time is up?