A Greek librarian's assistant named Phanocles and his sister Euphrosyne arrive at the villa of the Roman Emperor, having been forced out of their previous life of because of Phanocles' inventions, which drew scorn and allegations of black magic. Phanocles shows the Emperor a model of his design for a
steam-powered warship. The Emperor has no interest in it, but is delighted by the potential of the steam
pressure cooker, which Phanocles learned of from a tribe "beyond Syria". Mamillius, the Emperor's grandson, has no interest in either but falls in love with the veiled Euphrosyne when he sees her eyes. Phanocles is given the funds to build his warship, which is named
Amphitrite, and a second invention – a gunpowder artillery weapon later called the tormentum – in exchange for building the pressure cooker. The first version of the pressure cooker goes wrong, killing three cooks and destroying the north wing of the villa. Meanwhile, word of
Amphitrite construction reaches the Emperor's heir-designate, Posthumus (see
Postumus (praenomen)), who wrongly sees it as part of an attempt to put Mamillius in his stead. He leaves the war he was fighting to return to the villa and force the matter. On the day of
Amphitrite demonstration voyage, Mamillius and Phanocles are nearly killed and the ship's engine, called
Talos, is sabotaged, destroying several of the returning Posthumus's warships and most of the harbour through fire. It is revealed that the assassination attempt and sabotage were the work of enslaved rowers worried that the steam engine would make them redundant. The military have similar concerns about the impact of gunpowder on warfare. In the final section, Mamillius has become heir. Over steam-cooked trout, the Emperor tells Phanocles that he has decided to marry Euphrosyne himself to avoid embarrassing Mamillius, as he has deduced that the reason she never takes off her veil is that she has a
hare lip. Phanocles talks to the Emperor about his idea for a compass to solve the issue of navigating without the wind and reveals his final invention: the printing press. The Emperor is initially excited but becomes terrified by the prospect of vast amounts of bad writing that he would be obliged to read. To be rid of Phanocles and his dangerous ideas, he makes him
Envoy Extraordinary and Plenipotentiary to China. ==
The Brass Butterfly ==