The story follows an ancient Roman architect named Lucius, who is having trouble coming up with ideas. One day, he discovers a hidden tunnel underneath a spa that leads him to a modern Japanese bath house. Inspired by the innovations found there, he creates his own spa, Roma Thermae, bringing the modern ideas to his time.
Each subsequent chapter follows Lucius facing some sort of a problem, just to be swept to Japan once again. He visits modern bath houses, personal baths, waterparks, fertility festivals, and even zoos. There, he always happens to find the inspiration to solve the exact problem he has been tasked with.
Lucius Modestus (ルシウス・モデストゥス, Rushiusu Modesutusu), a Roman thermae architect who excels in designs ends up discovering modern Japanese baths, believing them to be the baths of 'flat-faced slaves'. He uses these ideas in his own work back home, eventually inspiring others with his discoveries. His endeavours sometimes lead to unfortunate consequences, including costing him his marriage, arousing suspicion of being Emperor Hadrian's homosexual lover, and destroying businesses.
Marcus Pietras (マルクス・ピエトラス, Marukusu Pietorasu), Lucius's friend and a marble sculptor. Oftentimes, he becomes Lucius's carver in his projects.
Hadrianus (ハドリアヌス, Hadorianusu), Emperor of Roman Empire. Initially commissioned Lucius of building a private thermae, then commissioned him for more baths.
Satsuki Odate, a 28 year old Japanese woman who is both the town geisha, and an expert in Roman history. She is also a polyglot, can speak and translate Latin. Fell in love with Lucius.
