The player controls a human child and explores the Underground, a vast subterranean landscape where they must battle monsters and solve puzzles to progress.

[citation needed] During battles with monsters, the player controls a heart representing the human's soul to evade bullet hell attacks from their opponents.

Selecting FIGHT causes one or more shooting targets to appear instead of the elliptical pattern from Undertale, but damage is still dealt by timing button presses.

Many years later, Asgore, the king of the monsters, has collected five of the seven human souls needed to break the barrier to the surface world.

Falling through a crack in the Ruins, they meet Flowey, a sentient flower, who teaches the player about the game's mechanics.

They encounter numerous monsters, such as Dalv, a recluse living in the Dark Ruins; Martlet, a member of the royal guard stationed in Snowdin Forest; Starlo, the sheriff of the Wild East; Ceroba, Starlo's childhood friend and the wife of a deceased scientist, Chujin; and Axis, a hostile robot residing in the Steamworks.

Most monsters engage Clover in combat, giving them the choice to either fight and possibly kill them or show them mercy.

During the final battle against Ceroba, Clover learns that Kanako fell down after requesting that her mother test the serum on her.

Martlet escorts Clover to the castle's throne room, and the game ends with Asgore reluctantly claiming their soul in battle.

During the game's credits, Dalv, Martlet, Starlo, and Ceroba float Clover's hat and gun down a raft leading to Waterfall's garbage dump.

The game's tone changes significantly during this route, with the soundtrack becoming more ominous, monsters disappearing from the Overworld, and most major fights becoming more intense and difficult.

Before they reach Asgore's castle, Martlet tries to stop them by transforming into a powerful being with a serum from the royal lab, but is defeated and killed.

[9] In response, Fox stated that he had not given recent permission but criticized Wolff's handling of the situation as "extremely unprofessional."

[10] Undertale Yellow was highly anticipated by YouTubers and received generally positive reviews upon its release,[11] garnering around 350,000 downloads in its first month.

[12] Before its release, Dominic Tarason of Rock Paper Shotgun declared Undertale Yellow to be his "favourite unfinished fan-game".