There is something very amiss in the finances of Michaelhouse. Despite a new influx of well-heeled students, there is an acute lack of funds. It is only when the Brother in charge of the account books dies unexpectedly that some sort of explanation is revealed: he has been paying for goods the college has never received. Although shocked by this fraud, Matthew is more concerned with the disappearance from his herbarium of a quantity of pennyroyal, a preparation known to cause a woman to miscarry, and a pregnant visitor to his sister's household has died from an overdose of pennyroyal. When Matthew learns that it was her husband who had received Michaelhouse's money for undelivered goods he exposes a very treacherous vein of deceit.