n the game, players control a human child who has fallen into the Underground, a large, secluded region underneath the surface of the Earth, separated by a magic barrier. The player meets various monsters during a quest to return to the surface, mainly through the combat system; the player navigates through mini-bullet hell attacks by the opponent cyclically, and can opt to spare monsters instead of attacking and killing them. These choices affect the game, with the dialogue, characters, and story changing based on outcomes.
Fox developed the entirety of the game independently, including writing and composing the score, with only additional art created by other artists. The game was inspired by the Mother and Mario & Luigi role-playing series, the bullet hell shooter series Touhou Project, and the British comedy show Mr. Bean. Undertale was initially meant to be two hours in length and released in mid-2014, but development took over three years, effectively extending the campaign and delaying the release.