When renowned archaeologist Professor Roderick Ainsworth unearths an ancient Roman temple of Mithras in London's Clerkenwell, his career is set to be crowned with a knighthood. Then a young analytical chemist is murdered in the temple, and circumstances suggest a ritual murder.

On the same day, a prosperous manufacturer is found slaughtered at Carshalton. Again, evidence points to ritual sacrifice. Has the discovery of the Mithraeum revived a dormant cult of Mithras?