A Roman engineer keeps tumbling into modern Japan and returning with increasingly unhinged spa technology that somehow makes perfect sense. The show treats this absurd premise with complete sincerity, letting the comedy come from Lucius's genuine confusion about Japanese bath culture instead of winking at the audience. You're laughing because a man obsessed with thermae genuinely believes onsen water heaters are witchcraft, and by episode three you're agreeing with him.

Lucius Modestus is a proud bath architect in ancient Rome who has run dry on new ideas and is out looking for work. To help him unwind, his friends bring him to a crowded public bath, where he ducks his head beneath the water and finds a hidden passage leading to a bathhouse in present-day Japan. He begins popping up there at random, and each trip hands him bathing innovations far beyond his era. Back in Rome he tries to rebuild what he saw, usually with rougher results given the materials he has, yet his inventiveness might be enough to impress the public and win back his standing. The result is a light comedy with a slice-of-life feel, built on a Roman marveling at Japanese bath culture.