A broken heart. A forbidden chord. And a direct line to the Lord of the Underworld.

Future concert pianist Francis Philbin didn't mean to start the apocalypse. He just wanted to win the heart of the beautiful Christine Chagnay. After his romantic mixtape is crushingly rejected?partially because Christine doesn't own a cassette player?a heartbroken Francis seeks a different kind of inspiration.

Turning from Bach to the dark side, Francis is introduced by his metalhead best friend to the Diabolus in Musica, the infamous Devil's Triad. Banned by the church centuries ago, the tritone is said to invoke pure evil. Francis employs it in his latest composition, and the response is immediate: he summons Lucifer himself.

The suave, terrifying Devil reveals that Francis's new masterpiece is the literal musical key needed to unlock the gates of Hell. Francis has until midnight to finish the song and unleash Armageddon, or watch his family flayed alive. With the world beginning to crack around him, Francis must find the ultimate counter-melody hidden within the riddles of a classic song to defeat Satan and close the abyss before the final curtain falls on humanity.