Juliet and Vivian Lowe are twin sisters attending their last year at Lindberg Academy, a prestigious boarding school for the performing arts.

The staff announce that Moira will be replaced at the senior school showcase and auditions will be held to see who will play in her place.

After finishing practicing, Juliet hears violin music from Moira's room that stops as soon as she enters, and finds a sun symbol etched into the wall behind a curtain, that matches the one on the notebook.

Juliet uses the opportunity to be assigned to the prestigious Dr. Cask, who is also mentoring her sister, and later, finds bloody tampons in her shelf.

Juliet looks up Moira's suicide on the internet, and learns that her mother fell off a ski-lift and went into a coma and her father burnt down their house and himself.

She falls into a trance and through automatic writing produces an illustration which demands sacrifice, and in a panic, she burns the entire notebook.

She starts to jump but sees herself in the auditorium having finished her playing, receiving a standing ovation and the approval of her sister.

In September 2019, it was announced that Sydney Sweeney, Madison Iseman, Jacques Colimon and Ivan Shaw had joined the cast of the film, with Zu Quirke writing and directing in her directorial debut.

[2] In October 2020, the rest of the cast was announced, including Julie Benz, Brandon Keener, JoNell Kennedy, and others.

The website's critics consensus reads: "Nocturne's thought-provoking themes find themselves at odds with its genre ingredients, resulting in a mild blend that isn't quite pulpy enough.

[5] Jordan Mintzer of The Hollywood Reporter called the film "A rather familiar tune",[6] while Ryan Lattanzio of IndieWire wrote that "[Sydney] Sweeney delivers on the promise of her turn in "Euphoria", but deserves better than this "Black Swan"-lite tale that's more half-baked ripoff than homage".