Romagna, Italy 1502.

Fleeing from the murderous brigand Sandino, the boy Matteo is rescued by the companions of Leonardo da Vinci. At this time da Vinci is in the employ of Cesare Borgia. Personally ambitious, cruel and ruthless, with a strange attachment to his sister Lucrezia, Cesare punishes without mercy those who oppose him or who get in his way. So Matteo is plunged into danger, for he carries with him a secret, a secret that the Borgia and their enemies the Medici would kill to discover.

Taken into the household of da Vinci, where he is befriended and educated, Matteo witnesses the Maestro at work on his various projects and accompanies da Vinci as he examines cadavers in his quest to find the source of the soul. This is an historical story of the Renaissance with all its beauty and art, combined with the harshness of torture and public executions. But The Medici Seal is also the personal story of Matteo, a boy becoming a man, whose mysterious background is slowly revealed as he becomes caught up in the intrigues of the Medici to regain control of Florence.