In Rome in the year 128 AD, a Roman named Lucius Modestus is having trouble coming up with ideas for architecture. While contemplating in a bathhouse, he discovers a strange drain than sends him to a bathhouse in Modern Japan. While confused by his surroundings, he becomes amazed by some of their innovations, such as fruit flavored milk. Taking these ideas back to Rome, he incorporates these ideas into a new bathhouse to great success.

A year later, Lucius is commissioned by a sickly man to build an outdoor bath near Vesuvius. While inspecting a heated well, Lucius is once again sent to modern Japan, this time winding up in a hot spring, where he gets to sample boiled eggs and sake. These innovations help nurse the old man back to health.

As Lucius hears fancies about a bath closer to home, he once again ends up in modern Japan, this time in someone's bathtub. Mistaken as a foreign caretaker by the house's owner, Lucius comes to learn about more modern conveniences.

After he builds a bath using this knowledge, his invention gets round to the Emperor Hadrian. There, Lucius suddenly finds himself with a strong urge to use the toilet, however he is summoned by the Emperor and has to bear it. He is to become the Emperor's new architect but he feels he is not worthy of such an honor. Not a moment later he is sent once again to modern Japan, this time in a toilet-showroom, where he sees sea life footage from a digital TV (which he interprets as an aquarium) and a modern bidet toilet. He then uses this knowledge to create these for the Emperor.

Lucius is sent to a hot spring near Vesuvius to build some public baths, managing to enlighten some smelly robbers living there. When he slips up, he once again enters modern Japan, discovering a city filled with public baths.

In Japan, Lucius samples the delights of an amusement stand and a ramen shop before bringing some souvenirs back to Rome, where he builds a village full of baths.