21: Two One is a Japanese murder mystery visual novel developed by BasiL, it was first released as an adult PC game for Microsoft Windows on May 25, 2001.

Gameplay in 21: Two One offers branching plot lines, and a multi-choice navigation, where the player can attain multiple endings.

21: Two One is a romance and murder mystery visual novel where the player assumes the role of Takuya Kirishima, the main protagonist of the game.

Throughout the game, the player will encounter CG artwork, which replaces the background art and character sprites.

For example, in several instances in the game, when the player is conversing with another character, they will be given the following options: "Trust", "Normal", and "Suspicion" (comparable with L.A.

[1] Throughout gameplay, the player is given multiple options to choose from, and text progression pauses at these points until a choice is made.

The player will also encounter various background music (BGM) throughout the game, different BGMs will play depending on the atmosphere or tone of the scene.

[2] Takuya also works alongside Momiji Kikyou (汽京 紅葉, voiced by: Kotono Mitsuishi), a medical doctor.

[2] Yuina Tachibana (橘 唯奈, voiced by: Satsuki Yukino) is another nurse who works in the hospital, her circumstance is similar to the protagonist, after the loss of her brother, she decided to become a doctor.

[2] Other characters include Konoha Ichinose (一ノ瀬 木葉, voiced by: Omi Minami), a mischievous little girl who is patient at the hospital,[2] and Kasumi Mihara (三原 香澄, voiced by: Mayumi Iizuka), Konoha's energetic friend who often visits her in hospital.

[2] One day, a murder is committed at the hospital, and since the event, various people start suffering from a strange, unknown disease.

[3] 21: Two One is BasiL's second project, after their first visual novel Bless: Close Your Eyes, Open Your Mind.

[10] It was written by Nikaido Kageyama, and illustrated by Aoi Nishimata, one of the main character designers for the game.