Ever since Deltarune was released, one of the most persistent questions surrounding the game has been its relationship with Undertale. Deltarune is not a sequel, nor is it simply a retelling. Its world, characters, and events are different, yet countless details seem deliberately familiar. The more we examine these similarities, the more convincing another possibility becomes: Deltarune and Undertale may be parallel stories that mirror each other in opposite directions. The most fundamental difference between the two games may already reveal this relationship. In Undertale, the protagonist falls from the surface into the Underground. The entire journey is essentially a movement toward the surface: we pass through the Ruins, Snowdin, Waterfall, Hotland, the CORE, and finally New Home, attempting to escape the Underground. Deltarune begins from the opposite position. Kris already lives on the surface, in Hometown, yet Dark Worlds repeatedly appear beneath or inside the ordinary world. As the story progresses, the characters seem to move deeper and deeper toward something hidden underground. If Undertale's geographical progression is reversed, an intriguing correspondence appears. Its sequence of major regions can be read as New Home CORE Hotland Waterfall Snowdin Town Snowy Ruins while Deltarune appears to be progressing through these environments in approximately the opposite direction. Chapter 1 therefore corresponds remarkably well with New Home. Both involve a castle, a king, and a confrontation with a ruler at the end of a journey. The difference is that New Home is near the end of Undertale, while Chapter 1 is where Deltarune begins. The same location therefore occupies opposite positions in the two narratives. Chapter 2 then resembles the CORE. Both are heavily connected to technology, machinery, and artificial environments. More specifically, both contain a NEO form: Mettaton NEO in Undertale and Spamton NEO in Deltarune. Both are incomplete or abandoned transformations, both possess striking wing-like structures, and both end in explosive destruction. The similarities are not merely visual; both stories use the NEO form to represent an artificial attempt at becoming something greater. Chapter 3 corresponds to Hotland even more clearly. Undertale's Hotland contains Alphys's technology, Mettaton's television programs, quizzes, challenges, and the famous Watercooler. Deltarune's Chapter 3 places Kris and Susie inside a television-centered world where they are repeatedly subjected to shows and challenges. The television-host structure is so specific that it strongly resembles Hotland's narrative identity. Chapter 4 brings us to Waterfall. This is particularly significant because Waterfall is one of Undertale's major sources of ancient lore. Its inscriptions reveal the Prophecy and explain the history of humans and monsters. Deltarune's Chapter 4 similarly revolves around a Church Dark World heavily connected to prophecy and ancient history. Justice is another striking connection. Undertale gives us Undyne, the Spear of Justice, while Deltarune gives us Gerson's Hammer of Justice. Gerson is himself a legendary warrior and Undyne's mentor. Chapter 4 also introduces the Green SOUL, creating another parallel with Waterfall's association with the Green SOUL. Then comes Chapter 5. Its connection is not specifically to the entire Snowdin region, but to Snowdin Town itself. Both are towns populated by monsters and filled with everyday activities rather than being purely hostile environments. Deltarune's Chapter 5 also contains Hometown's festival, while Snowdin has its own festive winter atmosphere and Noel-related decorations. Even the music provides a small but interesting connection: Undertale's Shop theme is heard in Snowdin's shop, while Deltarune uses Shop 3 in its corresponding shop sequence. These parallels show that the stories are strongly connected. #deltarune #deltarunetheory #undertale #gametheory #deltarunechapter5 - @maru_ll