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. 21: Two One was later released for the Dreamcast by PrincessSoft on December 27, 2001. The game is the second installment by BasiL, and after the completion of 21: Two One, BasiL went on to create their third and final eroge, Cherry Petals Fall Like Teardrops, before forming a new company called Navel. The story follows a young man called Takuya Kirishima, who is a talented surgeon and works in a hospital with his childhood friend Seri Sakaki, however, one day a murder is committed at the hospital, and Takuya tries to solve the mystery. Gameplay in 21: Two One offers branching plot lines, and a multi-choice navigation, where the player can attain multiple endings.

21: Two One had good sales, premiering at No. 2 in the rankings, which took place in the second half of May 2001. The majority of the game’s characters were designed and drawn by Aoi Nishimata. A special fan disc was given out as a preorder bonus and includes an additional scenario entitled 210: Two One Zero. Other media has been published, such as a light novel and original soundtrack.