A young homeless woman is discovered dead in a freezing ditch by an agricultural worker. Pale and still undecomposed, she looks strangely peaceful, as if asleep, and in the mysterious and fractured drama to come, showing flashback episodes in her life descending to this grimly inevitable ending, she is often to be discovered sleeping rough in very much the same situation. As one witness says: “If they’re dead, it’s ugly; if they’re asleep, it’s cute.”

This is Mona, unforgettably played by Sandrine Bonnaire; she has evidently changed her name from Justine, to distance herself from a former wage-slave life as a secretary. She took off on the road to nowhere with her tent on her back, sleeping in fields or on roadsides, getting cash-in-hand jobs where she could, sometimes selling sex and sometimes getting raped: always defiant, faintly mutinous, seductive, uncaring, inscrutable. As she says: “Je m’en fous – je bouge” (“I don’t care – I move on”).

She is the star of Agnès Varda’s award-winning 1985 film Vagabond, or Sans Toit Ni Loi (No Shelter No Law) now being revived as part of a retrospective in honour of the director’s 90th birthday. This extraordinary film-maker – part of that remarkable and long-lived New Wave generation – is still working, still creating, but this is surely one of her greatest films: enigmatic, possessed of a cool artistry in its structural asymmetries and inconsistencies, and as gripping as any thriller.

An object of desire and envy … Vagabond. Photograph: Artificial Eye

Varda tells Mona’s story via testimonies of the various people she met: a technique that goes back to Citizen Kane and anticipates Carol Morley’s 2011 documentary Dreams of a Life, about the young woman who died in her London flat without anyone noticing for years. It is a story of someone who has effectively abandoned her identity as well as social status, an outsider, whose existence breaks cover in criminal acts. It reminded me of Rene Allio’s film of Moi, Pierre Rivière … or Cédric Kahn’s Roberto Succo. There is even something Balzacian in the rich spread of characters that Mona encounters.

The film pieces together what happened. Sometimes the witnesses speak to an off-camera interviewer just before the relevant scene, sometimes they appear long afterwards, as if to offer an afterthought, and sometimes they will turn in the middle of an encounter with Mona, break the fourth wall and directly address the camera. There appears to be no strict rule or procedural logic in this, and the witnesses seem to have no connection with each other apart from their acquaintance with Mona. But then it appears some of them are related in unexpected ways, a middle-class mesh of prosperity extending horizontally over poor Mona’s head.

Wherever she goes, Mona spreads unease. Very often, people secretly envy and admire her and feel discontented with themselves; they are jealous of her freedom and think that for all her poverty she is really alive. But not everyone thinks this way. A hippy and his girlfriend take a shine to Mona and give her some land to work and a caravan to live in. But then they are deeply irritated and disappointed when Mona shows no inclination to get out of bed in the morning. This man tells her that, for all the romantic notions, life on the road, with its grinding poverty, is a kind of living death.

Perhaps most complicated is Yolande (played by the Belgian character actor Yolande Moreau) who is maid to a rich old lady and whose uncle is the caretaker of her employer’s semi-derelict chateau. Yolande is one evening to discover Mona and a boyfriend asleep in this property, having broken in. She doesn’t inform on them, envying their happiness. They seem to her to be truly in love – in contrast to Yolande’s sullen relationship with her shifty, petty-criminal boyfriend. Yolande is fortuitously to encounter Mona again, and comes to feel very differently about her. Perhaps she can’t make up her mind whether she has an underdog bond with her, or if she is, in her heart, loyal to the ruling class.

It is the chronicle of a death foretold. Perhaps the most arrestingly mysterious episode comes when a stylish university academic (Macha Méril) encounters Mona and winds up driving her around, disgusted by her smell and yet also fascinated by her. Later, this same woman is almost electrocuted in a freak accident after she puts both hands on a poorly fitted electric light. Badly shaken, she tells her nephew that she saw her life pass before her eyes. Perhaps, in some strange sense, like the workings of divine grace, it is Mona herself who has electrified this woman, made her take stock of her own existence.

And so we watch Mona (or is it Justine?) stumble from place to place, from pointless guy to pointless guy. We have already seen her dead body. It makes her seem like a dead woman walking. But all these people she meets: aren’t they heading the same way? Aren’t they all dead people walking, talking, and training themselves not to think about the future? An unmissable film from this great director.