Undertale is a role-playing game that uses a top-down perspective. In the game, players control a child and complete objectives in order to progress through the story. Players explore an underground world filled with towns and caves, and are required to solve numerous puzzles on their journey. The underground world is the home of monsters, many of whom challenge the player in combat; players decide whether to kill, flee, or befriend them. Choices made by the player radically affect the plot and general progression of the game, with the player's morality acting as the cornerstone for the game's development.
When players encounter enemies in either scripted events or random encounters, they enter a battle mode. During battles, players control a small heart which represents their soul, and must avoid attacks unleashed by the opposing monster similar to a bullet hell shooter. As the game progresses, new elements are introduced, such as colored obstacles, and boss battles which change the way players control the heart. Players may choose to attack the enemy, which involves timed button presses. Killing enemies will cause the player to earn EXP (in turn increasing their LOVE) and gold. They can use the ACT option to perform various actions, which vary depending on the enemy. If the player uses the right actions to respond to the enemy, or attacks them until they have low HP (but still alive) they can choose to spare them and end the fight without killing them. For some boss encounters to be completed peacefully, the player is required to survive until the character they are facing has finished their dialogue. The game features multiple story branches and endings depending on whether players choose to kill or spare their enemies; and as such, it is possible to clear the game without killing a single enemy.
Monsters will talk to the player during the battle, and the game will tell the players what the monster's feelings and actions are. Enemy attacks change based on how players interact with them: should players choose non-violent options, enemy attacks are easy, whereas they become difficult if players choose violent options. The game relies on a number of metafictional elements in both its gameplay and story. When players participate in a boss battle on a second playthrough, the dialogue will be altered depending on actions in previous playthroughs.
Plot
Undertale takes place in the Underground, a realm to where monsters, once equal to humans, were banished after war broke out between them. The Underground is sealed from the surface world by an imperfect magic barrier, the only point of entry being at Mount Ebott. A human child falls into the Underground and encounters Flowey, a sentient flower who teaches them the game's mechanics and encourages them to raise their "LV", or "LOVE", by gaining "EXP" through killing monsters. When Flowey attempts to murder the human to take their soul for himself, the human is rescued by Toriel, a motherly goat-like monster, who teaches the human to solve puzzles and survive conflict in the Underground without killing. She intends to adopt the human, wanting to protect them from Asgore Dreemurr, the king of the Underground.
The human eventually leaves Toriel to search for Asgore's castle, which contains the barrier leading to the surface world. Along the way, the human encounters several monsters, including: the skeletons Sans and Papyrus, two brothers who act as sentries for the Snowdin forest; Undyne, the head of the royal guard; Alphys, the kingdom's royal scientist; and Mettaton, a robotic television host Alphys created. Some of them are fought, with the human having to choose whether to kill them or to show mercy; should the human spare them, they may choose to become friends. During their travels, the human learns the cause of the war between humans and monsters. Asriel, the son of Asgore and Toriel, befriended the first human child who fell into the Underground and was adopted by Asgore and Toriel. One day, the child killed themselves by eating poisonous flowers. When Asriel returned their body to the humans, they attacked and fatally wounded him, resulting in Asgore declaring war. Asgore now seeks to break the barrier, which requires him to collect seven human souls, of which he has six.
The game's ending depends on how the player resolved encounters with monsters. If the player killed some but not all monsters, the human arrives at Asgore's castle and learns that they also need a monster's soul to cross the barrier, forcing them to fight Asgore. Sans stops the human before their confrontation, revealing that the human's "LOVE" is an acronym for "level of violence" and "EXP" for "execution points." Sans judges the human based on the combined resolution of the encounters. The human fights Asgore, but Flowey interrupts them, killing Asgore and stealing the human souls. With the aid of the rebelling souls, the human defeats Flowey, falls unconscious, and awakens on the human side of the barrier; they receive a phone call from Sans, explaining the state of the Underground after the human's departure.
