York, 1363. Former captain of archers Owen Archer is sent into the household of master apothecary Nicholas Wilton after medicines prepared in his shop appear to have killed both an unknown pilgrim and a nobleman. Posing as an apprentice, Owen must learn enough of the trade to decide whether the deaths were caused by error, substitution or deliberate poisoning.
Much of his knowledge comes from Wilton's capable wife, Lucie, whose position in the household makes her both an indispensable ally and a possible suspect. As further deaths increase the pressure for an arrest, Owen follows the movement of remedies, money and secrets through medieval York and discovers that the motive reaches beyond the apothecary's shop.