It is drawing near to the end of term, and the University at Cambridge is in turmoil over the opening of a new Common Library. There is an attack on one of the masters at a meeting to discuss the matter, and a body is found floating in the pond in the library's garden on the eve of its opening.

Meanwhile, there are rumours of a large force of dangerous smugglers lurking in the Fens. Aided by their friend Sheriff Tulyet, Bartholomew and Michael must thwart the invaders before the Feast of Corpus Christi the following week. To fail might mean the destruction of the town..