CW: This review contains discussion of bulimia and self-harm.

Pablo Larrain's Spencer is a dizzying, dazzling, and tragic portrait of the late Diana, Princess of Wales. Described in the opening subtitle as “a fable from a true tragedy”, Spencer is an imagining of Diana's visit to Sandringham during the Christmas of 1991, a year before her divorce from Prince Charles was announced to the world. Larrain spells it out from the start: this is not a biopic, this is not a retelling of events: this is fiction. Achieving historical accuracy is not within the film's intentions, although this hasn't stopped critics (and “royal experts”) from picking it apart and examining it under such a lens.

Our story opens on Christmas Eve. Privately, Diana (Kristen Stewart) and Charles (Jack Farthing) are separated, but Diana is still obliged to keep up appearances and attend the yearly tradition of Christmas at Sandringham with the in-laws. Our heroine's first line — “Where the fuck am I?” — epitomises the disaffected streak present in this incarnation of Diana; the last decade has made her a woman at the end of her tether, teetering on the edge of a nervous breakdown. This is not your mother's Lady Di.

It's these apects of her characterisation which make Kristen Stewart such a perfect fit for the role, particularly in this specific period of tension in Diana's life where she is growing increasingly detached from the royal family, but is still duty-bound (and bound by her children) to play her part. As one of the few non-Brits in the cast, Kristen already feels like an outsider; when we see her sat around the dinner table it's not only her nervous physicality and reluctance to eat that sets her apart, but the fact that it is Stewart who is playing her.

Stephen Knight, perhaps best known for creating the BBC period crime drama Peaky Blinders, is responsible for the screenplay, which is steeped in metaphoricity. The dialogue is, at times, deeply poetic and imbued with a bittersweet resonance, founded in the dramatic irony underpinning the whole tale. Of course, we know how this story will end, we know the fate of our heroine, and this knowledge makes Knight's script play out like a Greek tragedy.

Larrain places Diana's bulimia at the heart of the story. Upon her arrival to Sandringham, she is reluctantly weighed by the new equerry (Timothy Spall) — it's part of a (real) Christmas tradition, dating back to Edward VII's rule of weighing before and after festivities to ensure everyone has been “well-fed”. In Spencer, tradition is claustrophobic. Diana's bulimia accentuates the austereness that is life as a member of the royal family — inside of Sandringham she is under constant surveillance and held to rigid routine — two things that bulimia hates. Bulimia demands secrecy, not being interrupted during every toilet break.

Kristen Stewart's performance as Diana is intoxicating. She has been praised for her surprising likeness to the late princess, and she certainly embodies Diana's ability of generating astounding empathy in her beholder. Even opposite more seasoned acting greats like Timothy Spall and Sally Hawkins, Stewart still manages to command your attention. Stewart masterfully handles this multifaceted performance of a woman living a life split between two different identities — Diana Windsor and Diana Spencer — and the pivotal period of time where the cracks of her royal facade are starting to show.

Jonny Greenwood's score helps keep the tension embodied by Stewart, packed with foreboding minor key strings that manifest in the viewer the same constant anxiety Diana is feeling. With his work on There Will Be Blood, Phantom Thread, the soon-to-be released The Power of the Dog, and now Spencer, Greenwood is certainly one of the best and most exciting film composers working today.

Surreal, visually superb, but most of all tragic, Pablo Larrain has produced a masterpiece with Spencer.

Spencer is out now in cinemas.