Angel's Lullaby takes place during the game's opening introduction, back during the Human and Monster War. Expanding on the lore, meta narrative and core themes of the original games, Angel's aims to enrich the Undertale world by mixing both the old and new. Combining both an open-world and linear story-line, Angel's is an ambitious fangame with a large story to tell. There's three main "Acts," each with it's own main area that follows a story.

A long time ago on a distant unknown land... an island of Monsters and Humans lived in peace. Then one night the Hoppe's Kingdom cried out in despair. The royal's child had been taken by monsters. War erupted with a sudden strike onto Mythosa with the Humans victorious. However, the war could not bring back the one they lost. Knowing of their new power, humans feared monsters would retaliate. Desperate, the king relied on the seven wise to call onto their god, The Angel. Today, you wake up, encased in the body of a freshly undead human. The fate of Humans and Monsters lay within your hands. Who's side will you choose?