The Undermill is the first level besides Fur Fighter Village you can play through. It is essentially a training level run by General Bristol and Sergeant Sternhauser to help the player get used to each individual Fur Fighter. There are no enemies in the level, and the babies contained within were found by Bristol himself, making them the easiest babies to obtain in the whole game. This is also where the Teleportation Device is located.
As soon as you enter the Undermill you will be greeted by General Bristol, who, along with Sergeant Sternhauser will guide you through the level from start to finish. He will give you explanations on how the Telepoint system works, as well as get you started with some babies and Gold Tokens.
After being introduced to Sergeant Sternhauser and the use of Pet Yums, you will be moving across some cogs to find some flour bags waiting. Here, you can practise your aim, before moving on to test out Roofus' Digging Holes. Once that's done, a new room suited for Bungalow to test out his high jumps can be found, with crates of varying sizes scattered around. From here, another, similar room for Juliette can be encountered, with a large block displaying Cat Scratches for her to continue the level.
Tweek's area is next, where he will learn how to glide, allowing him to reach Chang's telepoint. A couple of small vents can now be entered, though only one will get Sternhauser's attention. Move through this blue vent to enter a series of new vents which can be exited and walked on. A ladder is provided for those who fall down.
After progressing further here, you will be presented with the final area of the level. After having a quick chat with General Bristol, you will need to locate the nearby Rico telepoint and practise Swimming. Once you swim through, General Bristol and Sergeant Sternhauser will wish you luck on your journey, and you can backtrack to the entrance (which also serves as the exit).
- It is possible to skip playing as Tweek and Juliette during their individual training sequences by using Bungalow's High Jump (or if you're playing as Juliette, she can still avoid using Tweek by jumping straight to Chang from Sternhauser's spot). The same can be done for Bungalow's training sequence, as the walls on the side have indents that can allow Roofus to reach Juliette.
- In the Dreamcast version of the game, Juliette can climb the barriers near Roofus' area.
- As a result, there is a secret, unused area that Juliette can reach.
- The Dreamcast version of the game is the only version that forces you to progress slightly through this level before General Bristol gives you the Subway Token.
- This is the only regular level not to have a Time Attack mode in any version of the game, despite having a Time Attack score.
- The Undermill is the only level with no enemies to be found (excluding Hubworlds).
- If you avoid speaking to Bristol in the PS2 version when he talks about Gliding or Cat Scratches, he will be glitched to respawn in the spots where he talks about those abilities. He will not start a conversation if you walk into him, regardless if you're playing the correct fighter or not.