The 40K Ending is unlocked when you reach a score of at least 40,000 or greater in a single game. The player will also be greeted with a Red-Eyed Irisu Kyouko when the game ends, which is identical to when you click off the game while playing. This will unlock a small cat at the lower right corner. Clicking on it will play a flash of images from the Metsu Mode unlockable notes, and words presumably said by Uujima, before unlocking the game's Metsu mode. The photo in the directory is also changed, with Age Hatori's face being scribbled out in white.
A new file will also appear in the directory when you reach a score of 40,000 or greater, called "Lately I've Been Taking Cough Medicine Without Restraint.txt". The text document further describes Irisu's life as a student, her opinions on the world, how she met Uujima and formed feelings for him while referencing the death of U-Tarou, how she read Uujima's notebook and found the suicidal cat drawings, why she hates Age Hatori due to comparing her to a cat, and calling Edogawa Takeru annoying. This leads to her learning about their plan to take Uujima on a trip with them, and plotting to kill Age and Edogawa by becoming their friend, being allowed to come on the trip, and being able to make decisions about the trip, even if she hates speaking to them. Later, she continues writing, possibly having a change of mind and realizing her plan was flawed, and believing Age is the only one who should die, as Edogawa is "harmless". During Irisu's writing, she takes several packets of powdered cough medicine repeatedly, showing her issues with having extremely harsh and painful coughing fits, but also hinting at an addiction to the medication, as she mentions how she has taken 30 packets of it, and how she feels hazy, with a drunken mind as thoughts spiral through her head, probably further emphasizing her mental health issues. At the very end of the document, it shows Irisu's her final intention of what she plans to do on the trip.
A glitch or oversight is caused when you view Metsu Mode before obtaining the 100,000 Score Unlockable image of suicidal rabbits, preventing the player from accessing it even if they go back to achieve the score. To avoid this, players must achieve a score of at least 100,000 or greater in a single game in Normal Mode before clicking on the cat to access Metsu Mode.
Metsu Mode is also accessible with a secret code. By pressing Up, Up, Up, Up, Right, Right, Down, a sound will play, and the small cat in the lower right corner will appear to allow access to Metsu Mode, while also giving access to the document written by Irisu in the game's directory.