Although he was watching closely when the mummer was poisoned, it took Gil Cunningham several days and three more poisonings to work out how it was done. Danny Gibson and Nanty Bothwell, rivals for the affections of Agnes Renfrew, the apothecary's pretty daughter, are also good friends. When they both take part in the festive play at the house of Gil's sister Kate, it ends in Danny's death, apparently by poison from his friend's flask. So was it deliberate, and if not, why won't Nanty defend himself? Why is Agnes's eccentric brother Nicol so insistent that Nanty had the wrong flask, and why do none of the apothecaries in Glasgow recognize the poison it held? Gil, convinced Nanty is innocent, sets out to answer these questions and finds himself enmeshed in the tensions of the Renfrew household and the tangled relationships among the apothecary houses. And then a second and third death confuse matters further still, and bring Gil's wife Alys into the investigation.