Leonardo da Vinci finished The Last Supper in the late 15th century on the refectory wall of Santa Maria delle Grazie in Milan, painting in oil and tempera on dry plaster rather than true fresco, a choice that began ruining the work almost immediately. The film follows conservators, art historians, and technicians tracing five centuries of damage: flaking paint, a doorway cut through Christ's feet, wartime bombing that left the room roofless, and repeated botched restorations that painted over Leonardo's own brushwork. Infrared reflectography and X-ray scans let researchers look under the surface for what survives of the original layer versus later overpainting. Interviews cover the decades-long modern restoration effort, led by Pinin Brambilla Barcilon, that stripped away centuries of retouching to recover whatever original material remains. The painting's fame and fragility sit side by side throughout: millions view it each year in timed fifteen-minute slots, while conservators debate how much of what they see is actually Leonardo's hand.