The story begins with Marianne, a young painter who is teaching portraiture to a group of students. One student, without Marianne's permission, digs out a portrait that Marianne painted years ago. She's shocked to see it -- the image of a woman, her back towards her creator with fire burning the bottom of her dress. Upon being asked what it's called, Marianne replies, "Portrait of a Lady on Fire." Flashback to Brittany, near the end of the 18th century, Marianne is commissioned to capture the likeness of a young woman named Héloïse, the painting meant to be a gift for her future Milanese husband. But Marianne is told that she must paint Héloïse in secret because she refuses to pose, in an act of defiance towards the marriage she's being forced into.
The portrait session begins as Héloïse walks ahead. Her hood slowly falls with the pace of her step and a gust of wind, her blond head of hair revealed. She runs with Marianne in pursuit. Héloïse stops at the edge of a cliff, and there's a moment where you think she's going to jump. She turns around. "I've dreamt of that for years," she says. "Dying?" Marianne asks. "Running."
There's a scene in 1933's Queen Christina where Greta Garbo, after spending the night with her lover, gets up and begins pacing around the room. She begins to caress every object, looks upon her lover's reflection in a mirror, and grasps onto the bedpost. "I have been memorizing this room," she tells him. "In the future, in my memory, I shall live a great deal in this room." That important intimacy of memory is a big part of Portrait of a Lady on Fire, too. Sciamma implies heavy use of voyeurism, as Marianne must slowly and secretly commit Héloïse to memory, often closing her eyes for a few seconds to conjure her likeness. And as the artist was observing her muse, the muse was watching back.