The clue to a murder in the art world of contemporary Madrid lies hidden in a medieval painting of a game of chess.

In a 15th-century Flemish painting two noblemen are pictured playing chess. Yet two years before he could sit for the portrait, one of them was murdered. In 20th-century Madrid, Julia, a picture restorer preparing the painting for auction, uncovers a hidden inscription in Latin that points to the crime: Quis necavit equitem? Who killed the knight? But as she teams up with a brilliant chess theoretician to retrace the moves, she discovers the deadly game is not yet over.