The plate-glass, concrete jungle of Euphoria State University, USA, and the damp red-brick University of Rummidge have an annual exchange scheme. Normally the exchange passes without comment.

But when Philip Swallow swaps with Professor Zapp, the Fates play a hand, and the two academics find themselves enmeshed in a spiralling involvement on opposite sides of the Atlantic. Nobody is immune: students, colleagues, even wives are swapped as the tension increases Finally, the cat is let out of the bag with a flourish that surprises even the author himself.