The Lab is a large white and red building with a pale green interior. There are two floors on the lab, the ground floor being Alphys's workplace, and the second floor being her room.
On the ground floor, there is a large screen where Alphys monitors the protagonist, a fridge with a supply of Instant Noodles, a messy desk with her computer, and a "washroom" that leads to the True Lab. Alphys's obsessions and hobbies, including comics, anime figures, an ice cream machine, etc. litter her room. On this floor, there are unopened letters from many monsters, presumably from the Amalgamates' families.
The Lab is where the protagonist first encounters Alphys after escaping from Undyne. Here, Mettaton confronts them in a quiz show. After deterring him, the protagonist has their Cell Phone upgraded. Then they exit the Lab and proceed on their way towards the CORE.
The protagonist delivers a letter from Undyne to Alphys, who reluctantly opens it and mistakes the protagonist as the writer of the letter. She hangs out with them, Undyne and Papyrus before Papyrus sends her to the Lab and calls the protagonist to recommend they go there as well.
Checking the bin between the computer and screen reveals that someone sent an intimidating message to Alphys, simply saying "I KNOW WHAT YOU DID," written in a strange handwriting. The "washroom" is open and near the doorway is a note from Alphys addressed to her friends, telling them that she needs to face her mistakes and that "the truth" is past the door. The protagonist enters the door, which is an elevator that takes them to the True Lab.
The protagonist quickly passes through the Lab, scaring away Mettaton.
If the protagonist aborts their Genocide run at Mettaton NEO, it is implied that when Alphys evacuated the monsters, she brought them to the True Lab. Alphys tells them that she knew she had to do something about the protagonist's actions, even if it meant that the monsters would learn the truth about her.
- In the Windows version of Undertale, the monitor follows the protagonist's movements, whereas the screen displays a static image in Mac OS X version.
- It is possible to skip the entire scene with Alphys and Mettaton by interacting with the bag of dog food right when the scene begins. This has been dubbed the "Lab Skip."
- In Undertale v1.001, the hitbox of the bag of dog food was shrunk to avoid this from happening. However, because Toby Fox could not figure out how to replicate it, this did not fix it.
- Interestingly, calling Papyrus's Phone twice in the lab will now cause him to recall seeing Sans with said dog food bag. This may be an in-game reason to explain why the bag's hitbox has shrunk.
- The "Lab Skip" has been abandoned by the speedrunning community for the "Punch Card Exploit" seen here since it is a much easier alternative to pull off.
- The bathroom sign and the switch around the entrance to the True Lab have been moved in Undertale v1.001.
- A rather amusing glitch before v1.001 can be pulled off by calling Papyrus and Undyne while on a conveyor. This causes the text and portrait to trail off the screen.
- ↑ "YOU'RE IN HER ROOM, UNINVITED? WOWIE... THAT'S EXTREMELY CREEPY." - Papyrus, phone call in room room_fire_lab2. Retrieved from http://rawr.ws/undertale/calls
- ↑ "so far bugfixes include shrinking the hitbox of a bag of dog food" - Toby Fox. January 10, 2016. Twitter.
- ↑ "well it's at least harder to do now (?) i couldn't figure out how to do it so its probably ok if its just a speedrun thing" - Toby Fox. January 20, 2016. Twitter.