Young Enn, portrayed by Alex Sharp, alongside his close friends, stumbles upon a surreal gathering of teenagers who aren't from this world. These intergalactic visitors have come to Earth to partake in an enigmatic rite of passage, but this encounter ignites a wild crush in Enn's heart for Zan, played by Elle Fanning. Zan is not just beautiful; she’s a spirited and rebellious alien, and her fascination with Enn leads them both on a vibrant escapade through the dynamic punk rock landscape of 1970s London. Their unexpected connection seemingly triggers an epic showdown between the earthbound punks and their extraterrestrial counterparts.
During a raucous evening en route to a punk concert in the quiet suburb of Croydon, Enn, John, and Vic unintentionally discover what appears to be a unique house party filled with eccentric, latex-clad attendees. Gradually, they unravel the truth: this offbeat assembly is a congregation of aliens. As the night unfolds, Enn finds himself smitten with the mysterious and daring Zan.
At the helm of this unusual gathering is Queen Boadicea, magnificently embodied by Nicole Kidman, a punk veteran who manages the local punk venue. Among the alien ranks are PT Stella, represented by Ruth Wilson, and PT Wain, portrayed by Matt Lucas, alongside PT Waldo, played by Tom Brooke, who is revealed to be Zan's parent and one of the alien leaders.
Intrigued by the mundane yet fascinating life of a local, Zan pleads with Enn to introduce her to Earth’s experiences, as her alien itinerary is painfully dull. Defying the expectations of her kind, she escapes her colony to spend just 48 hours indulging in human adventures with Enn. He welcomes her into his home, where her charm quickly wins over his mother, Marion, played by Joanna Scanlan. But the serene moments are shattered when Waldo inhabits Marion’s body, pressuring Zan to return to her home. Nevertheless, Zan is resolute in her desire to embrace life’s offerings, declaring that the essence of their alien journey lies in experiencing the unexpected.
The plot thickens with the introduction of PT First, the alien supreme leader, played by Edward Petherbridge. As Waldo initiates a vote to decide Zan’s fate, it becomes clear that returning to the colony means abandoning her burgeoning relationship with Enn. Zan, an avid punk enthusiast, eagerly accompanies Enn to meet Boadicea, but miscommunication leads to a twist where Boadicea mistakenly elevates Zan to punk stardom at the local hangout. In a moment of collaboration, both take to the stage to perform a song addressing alien customs.
Yet, as the audience revels in their performance, Zan shares a terrifying revelation: her colony's leaders plan to carry out a dreadful practice involving the children. Confusion ensues as the extraterrestrial assembly arrives, and Waldo warns Zan about the peril of being left behind if she doesn’t return. As tensions escalate, First discloses that Zan is pregnant, complicating her choice further. With the clock ticking, and facing a heartrending goodbye, Zan makes the choice to go back to safeguard her race's future by voting to halt the pervasive child-eating ritual.
Fast forward 15 years, and we find Enn thriving as a graphic artist with a bestselling book to his name. While at a book signing event, he is approached by a group of teenagers who introduce themselves as Zan's children. Overwhelmed with nostalgia and emotion, Enn realizes the profound impact Zan's choice had on his life and reflects on the love they once shared.
