Two women are alone in a house by the sea. One of them talks and talks; the other has decided to stop speaking altogether. By the time Persona is over you are no longer certain which of them is which — their faces have slid together in the dark, and the film has quietly proposed that this was the point all along. Few movies promise so little on paper and deliver something so unnerving.

The setup could hardly be barer. Elisabet Vogler, a celebrated actress, falls silent in the middle of a performance and simply never speaks again; her doctor sends her off with a young nurse named Alma to recover at a house on a stony island shore. Two performers, one location, almost no plot in the ordinary sense. Out of that near-emptiness Ingmar Bergman built what a great many people still regard as the most radical film ever made by a director working at the very top of the medium.

He begins by taking cinema apart in front of you. Persona opens on a flickering reel of fragments — a projector’s arc lamp, a spider, a slaughtered lamb, a nail driven through a palm, a boy reaching toward an enormous out-of-focus face — before the story proper has even started. Midway through, the image itself seems to tear, scorch and burn, as if the film in the projector had caught fire, then gathers itself and resumes. Sven Nykvist shot all of it in a black and white so clean it feels surgical, and his most famous composition fuses one half of each woman’s face into a single impossible portrait.

It rests entirely on two actresses, and they are extraordinary. Bibi Andersson, as the talkative Alma, carries most of the words — including a long, plainly told confession about an afternoon on a beach that is among the most charged monologues Bergman ever wrote, delivered once and then repeated with the camera turned on the listener. Liv Ullmann, as the watching, withholding Elisabet, says almost nothing and dominates the film anyway. Persona was the beginning of her long partnership with Bergman, and it is immediately obvious why he never let her go.

The title is the Latin word for the mask an actor once held up on a stage, and the film treats identity as exactly that kind of borrowed object. Alma talks herself empty and starts to bleed into the silent woman she is meant to be caring for; Elisabet’s silence turns out to be a form of power, perhaps even of vampirism. Is one woman absorbing the other? Are they two halves of a single mind? Is the whole encounter a fiction the film is openly staging for us? Bergman refuses to settle it, and the refusal is not coyness — it is the subject.

Very little of later art cinema is untouched by it. Robert Altman’s 3 Women, David Lynch‘s Mulholland Drive, Darren Aronofsky‘s Black Swan and a long line of doubling, identity-dissolving dramas all trace back to this one film. It sits near the top of almost every serious poll of the greatest movies ever made, and critics and film schools have spent more than half a century picking through its eighty-one minutes frame by frame without exhausting them.

None of that makes it a comfortable watch. It is cold, frightening and deliberately unresolved — a horror film whose only monster is the self. What it offers instead is the rare sensation of an art form testing its own limits in real time and finding new ones. Released in 1966, it has not dated by a single day, because it was never trying to depict its present; it was trying to discover what a face, a voice and a camera can actually do to one another. The answer it arrives at is still genuinely unsettling. That is why it endures.