Lucius, an architect in ancient Rome, discovers a time portal that transports him to modern-day Japan. There, he becomes inspired by the country's advanced bathhouse culture and brings back ideas to improve the public baths of ancient Rome.

In Thermae Romae II, Lucius, a Roman architect, discovers a time-traveling bath and continues his hilarious and bizarre journey through different eras of Japan, while trying to adapt to the modern world. Faced with new challenges and heartwarming encounters, Lucius still finds a way to appreciate the wonders of public baths and bring a taste of the ancient Roman Empire to the present day.