In late 19th-century Naples, Professor Ernesto Cacciapuoti is a respected physicist with an extraordinary secret: in a hidden laboratory on the ground floor of an old palazzo on Via Toledo, he has built a machine capable of opening temporal gateways. The invention, which he simply calls "The Door," is the fruit of twenty years of research following the mysterious disappearance of his father Vincenzo in 1877.

When strange phenomena begin to manifest in the streets of Naples-objects disappearing, people swearing they've seen figures dressed in clothes from other eras-Ernesto realizes that his initial experiments are creating ripples in the fabric of time. Together with his young assistant Gennarino, a bright boy from the Spanish Quarters, he decides to attempt a definitive experiment.

But the activation of The Door attracts the attention of an ancient secret society, the Guardians of Time, led by the mysterious Master Salvatore. When they burst into the laboratory to stop the experiment, Ernesto and Gennarino dive through the temporal portal and are catapulted into the Naples of 1877.