At the May Day dancing at Glasgow Cross, Gilbert Cunningham, notary-in-training, sees not only the woman who is going to be murdered, but her murderer as well. Gil is a recently qualified lawyer whose family still expect him to enter the priesthood. When he finds the body of a young woman in the new building at Glasgow Cathedral he is asked to investigate, and identifies the corpse as the May Day harper's mistress and runaway wife of cruel, unpleasant nobleman John Semphill. Together with Maistre Pierre, the French master-mason contracted to work on the new building at Glasgow Cathedral, Gil asks questions and seeks a murder weapon in the lanes and yards of the city, dealing with Semphill and his household, mistress and men-at-arms, the burgh constable, householders and musicians, and the mason's educated and lively daughter Alys, whom he finds increasingly attractive. A second murder seems only to complicate matters and the trail leads Gil and his new friend Maistre Pierre from Glasgow to the Isle of Bute. Gil must pursue rumours of missing silver, conflicting accounts of an elopement, and a girl with a toothache while at the time confronting his growing reluctance to embrace his family's plans for him. The mason and his daughter offer a solution, and when the killer is finally exposed, justice strikes from an unexpected direction and Gil is left to resolve his own future.