The world of monsters has been ravaged by war with humans, leaving behind a devastated landscape. A child, who falls through a hole in the ground, finds themselves brought into this strange new world under the care of Toriel, a kind-hearted monster woman. As they journey through the underground, they will encounter various creatures that can be hostile or friendly.
Undertale is an action-packed Eastern RPG with real-time combat elements. Players explore areas, solve puzzles, and interact with non-playable characters (NPCs) in the overworld. When a battle begins, typically through a random encounter, the game shifts to a fast-paced combat screen. The child's heart, representing their soul, must be dodged as objects representing monster attacks appear on a small box. On the child's turn, they can choose to attack more directly using the fight command.
Players have the option to perform friendly actions or show mercy to opponents in battle, resolving conflicts without bloodshed. However, some encounters are more challenging to resolve peacefully than others. Certain actions taken by the player in the overworld may affect their ability to befriend certain monsters.
The game remembers key player actions across playthroughs, including their level of violence or peacefulness. This affects how NPCs treat the player and ultimately influences the story's outcome. Only one Undertale save file is available at a time, but players can reset to an earlier save or start a new game after specific actions are taken.