- Many different monsters, such as the Snowdin Canine Unit, can be found relaxing, playing cards, and talking.
- A hangout with Sans occurs here, and during the hangout, it is revealed that he is a well-known regular, even having a tab.
After sparing Papyrus and making it to the entrance of Waterfall, Sans asks the protagonist if they want to go to Grillby's, as he is on one of his "legally-required breaks." If the protagonist agrees, he tells them to follow him through a "shortcut," and proceeds down the path the protagonist would take to proceed in Waterfall, which "transports" him and the protagonist to Grillby's.
Upon arrival, Sans is greeted warmly by the many patrons as he takes a seat alongside the protagonist at the bar. He then asks the protagonist what they want, giving them the option of getting a burger or fries. Sans orders the same meal that the protagonist requests.
While waiting for the food to arrive, Sans begins to strike up a conversation, asking the protagonist whether they think Papyrus is cool. Grillby brings the protagonist and Sans the dish that the protagonist had chosen at the beginning of the discussion. Shortly after, Sans produces a bottle of ketchup and asks the protagonist if they want some. Once again prompting a choice by the protagonist, they can either choose "yes" or "no".
- If the protagonist says yes, they take the ketchup and attempt to add some to their plate. However, the cap falls off, and the ketchup spills onto the food, ruining it. Sans then allows the protagonist to have his food instead.
- If the protagonist says no, Sans keeps the bottle for himself and proceeds to chug the entire bottle in one sitting.
Sans then goes on about how Papyrus had begged Undyne to allow him to join the Royal Guard, and the "training" he was receiving from Undyne.
Afterward, he tells the protagonist that "i wanted to ask you something." All action in the room seems to freeze. All light focuses on the protagonist and Sans, and "Premonition" begins to play in place of "sans.". Sans then goes on about how Papyrus had been getting advice, encouragement, and even predictions from a talking flower, but brushes it off as he felt it was simply an Echo Flower being used to play a trick on Papyrus.
Afterward, Sans says that he needs to get back to his post, as the protagonist had "held him up" and asks if the protagonist is willing to pay the 10000G necessary for the meal, prompting yet another yes/no question. However, no matter the protagonist's response, Sans says that he is just joking, and tells Grillby to put it on his tab. He then begins to leave, start a sentence but then forget what he was going to say.
If the protagonist kills Papyrus instead of sparing him, the monsters complain that Sans is not showing up, and this scene never takes place. Additionally, “sans.” stops playing in Grillby’s, leaving the restaurant with no background music.
The entire building is completely empty, as all of Snowdin had been evacuated at this point. Because of Snowdin's evacuation, the hangout with Sans does not occur, even if Papyrus is spared.
Grillby's contains a couple of booths, tables, a bar and a broken jukebox. As mentioned above, it contains a variety of NPCs, from Big Mouth to Punk Hamster, including the Snowdin Canine Unit, if the protagonist spares them before reaching Snowdin.
- Papyrus states that Sans usually eats out every night, and only recently began attempting to cook. It can be assumed that Grillby's would be the "out to eat" that Papyrus was mentioning.
- There is a "fire exit" to the left of the bar, which cannot be used because the protagonist is "not made of fire." This restriction implies that the exit was made for the owner, Grillby, only. This joke is a play on real life fire exits.
- The presence of Lesser Dog is supposed to prevent the protagonist from reaching the fire exit. However, it is possible to clip through Lesser Dog's hitbox and reach the fire exit without killing it.
- Grillby is either mute, speaks a different language than other monsters, or is simply not talkative. This behavior is revealed after the True Pacifist Ending when Red Bird, who usually speaks for Grillby, says "I might not get to translate for Grillby anymore..." However, when speaking to Grillby, he says "................... good job."
- On the Genocide Route, the protagonist is able to walk on top of the counter by walking onto the seat that Punk Hamster usually inhabits and then walking left.