Code_18 is the fifth entry in the Infinity series, which also includes Never 7: The End of Infinity, Ever 17: The Out of Infinity, Remember 11: The Age of Infinity, and 12Riven: The Psi-Climinal of Integral. The game follows Hayato Hino, a high school student who receives voice messages from the future on his phone, and is stuck in a loop, repeating a month of his life over and over again. Cyberfront bought the rights to the Infinity series after its previous developer, KID, closed down. The game's writing was done by the group Run & Gun, while the sound production was handled by the band Milktub. While the development team had originally planned to use a theme of "escape from an enclosed space" similarly to previous Infinity games, it was changed due to concerns of the size of the game's audience, and was seen as a reboot of the series.