Envoy Extraordinary is set in Ancient Rome, in and around the Emperor's court. The aged Emperor lives with his favourite grandson, the seventeen year old Mamillius. The true heir, Posthumus, is governor of a distant province, but is jealous of Mamillius.
The normal Roman world is shattered by the arrival of the inventor, Phanocles. He brings many inventions that are far ahead of his time, including the pressure cooker, steam engines and explosives.