Doctor Parnassus leads a travelling theatre troupe that offers audience members the unique opportunity to step through a magical mirror into fantastical realms of imagination. Despite his ancient appearance, Parnassus is centuries old due to a deal with the Devil, Mr. Nick, granting him immortality in exchange for any child he might father on their sixteenth birthday. His daughter, Valentina, is nearly sixteen, and Parnassus is desperate to protect her from his impending pact with Mr. Nick. The troupe's fortunes begin to change with the arrival of Tony, a mysterious outsider with a murky past, whom they discover hanging from a bridge.

The troupe's performances incorporate Tony’s ideas, revitalising their show and attracting new audiences. However, the success comes with a price as Tony’s presence escalates the stakes of Parnassus's bargain with the Devil. Parnassus is then thrust into a race against time to save his daughter from his fate, engaging in a series of fantastical challenges. The narrative delves into themes of redemption, sacrifice, and the power of storytelling, all while navigating the blurry lines between reality and the imaginary worlds conjured by Parnassus’s mind. As the deadline of Valentina’s sixteenth birthday approaches, the lines between friend and foe blur, and Parnassus must confront his past decisions and the ultimate cost of his immortality.