Welcome to Skeleton City, the bustling metropolis of the afterlife, deep within the earth. Here, skeletons of the recently deceased arrive daily, pulled from their graves. Among them is Geoff, a newcomer struggling to make sense of this bizarre existence. His roof is fire, he can still get drunk without a stomach, and his girlfriend, Naomi, despite being skinless, still worries about her figure. Geoff knows one thing for sure: he needs to cheer up Naomi, and that means embarking on an adventure in this confusing, mundane, yet strangely familiar underworld.

In a world where humans coexist uneasily with recently freed monsters, you are Y/N, codename 'Monarch Butterfly', a top spy and killer for a shadowy organization. Your life of espionage takes an unexpected turn when a mission to find the elusive monster ambassador, Frisk, leads you into a dangerous encounter with a powerful skeleton named Sans and a startling revelation that connects your hidden world with his. Now, you're faced with a choice that could change everything: remain loyal to the Organization, or make a deal with a monster to uncover a truth far bigger than you ever imagined.