Deltarune hasn't been advertised or officially stated as a sequel to Undertale, though the game itself tells the player that it's meant for those who have already played through Undertale. This implicated phrasing is important to figuring out how Deltarune and Undertale are related.

Toby Fox's follow-up to the hit indie game Undertale released over 2 years ago in 2018, and though it was only the first chapter of the game, players have since spent the months in radio silence deciphering Deltarune's mysteries. The big question players will have after finishing Deltarune's first chapter is what happened at the end, but to fully understand it, players need to have first completed Undertale and understand its Pacifist and Genocide endings as well as who Chara is.

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Upon starting the game, the player will be asked to create a character and give it a name, but the game will immediately discard it, saying "no one can choose who they are in this world," not unlike the general meta tone used in Undertale, and is similarly important to the takeaway. Instead, the player assumes the role of Kris, whose appearance should remind the player of Chara.

In Deltarune, after going into an empty classroom with the school bully Susie to look for some supplies, she and Kris accidentally fall into a strange world. They're told by a mage named Ralsei that they're the two 'Lightners' who are destined to save this world, but Susie is desperate to return home. The two help Ralsei overthrow the king and seal the Eastern Fountain before returning to the real world. Confused, they return to an empty classroom with toys strewn about. Before parting ways, Susie says she wants to return to this strange world.

Kris can wander around town, where they'll see many familiar faces from Undertale. When Kris finally returns home, a cutscene will play where they rip their heart out of their chest, throw it into a cage, and look at the camera with Chara's glowing red eye and signature knife.

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While it has yet to be confirmed, there are some heavy implications that Deltarune is a prequel to Undertale, in some form or another, taking place before the war between humans and monsters. Though, some fans speculate that they take place in separate universes. Firstly, there are many instances of the game telling the player they aren't allowed to make choices, unlike Undertale. Additionally, Toby Fox has confirmed that there will only be one ending to this game, unlike Undertale's many. In the case of a prequel, the game would have to end in a way that coincides with Undertale's lore, so players wouldn't be able to change the outcome.

Again, Kris can meet a number of players in town who resemble monsters in Undertale, such as Undyne, who isn't missing an eye and doesn't yet recognize Alphys. This would imply that this is before the events of Undertale. Additionally, the monsters here are living on the surface. Calling back to Undertale's intro sequence, a war between monsters and humans took place that eventually led to monsters being sealed in a strange world underground. And who was the first human to fall into this world? Chara, also known as "the first human."

In Deltarune, Kris is the only human seen in the underworld and shares the same design as Chara, and knowing Toby Fox, this likely isn't a coincidence. Kris's adoptive brother is Asriel, who is currently off at college, being cared for by Toriel. A big revelation from Undertale was that Chara was Asriel's adoptive brother who eventually killed him. It's quite possible that the two are the same person, in some capacity, at least. Could the events of Deltarune be leading up to the war?

Finally, when Kris pulls their heart out with that sinister look in their eye, for a short moment, they can control the heart moving around in the cage. The heart has always represented the player in the Undertale universe, but it's as if Kris is saying "you don't control me," calling back to the idea that the player can't change outcomes in this game so they can tie into the events of Undertale. 

Deltarune Chapter 1 is free for PC, PS4, and Nintendo Switch, and Chapter 2 is currently in development.

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