Cambridge, 1896. Motherhood and private detecting don't easily go hand in hand, but even with two small children Vanessa Weatherburn still manages to indulge her passion for solving mysteries. When three sombre scholars knock on the door of her family home, Vanessa is presented with perhaps her most puzzling case yet.

Professor Gerard Ralston, Head of the History Department at King's College, London, has been shot dead in his study. As the only suspect left the building a matter of seconds before the shot was heard, and with witnesses testifying that no one left the building after the shot rang out, all are perplexed as to how the killer could have escaped. Vanessa must use all her logic and intuition in order to solve the paradox of a seemingly impossible murder.