Rome wasn’t built in a day, and neither was its baths. Lucius (Hiroshi Abe) is a bathhouse architect in 2nd-century Rome. Having difficulty coming up with a design that will please Emperor Hadrianus (Masachika Ichimura), he comes across a time-traveling portal while unwinding in one of his bathhouses. For reasons explained later, Lucius is promptly whisked away to a bathhouse in present-day Tokyo, where he marvels at the modern bath culture of the Japanese (whom he calls “slaves” and “flat-faces”). Taking ideas and inspiration from the showers, saunas, hot springs, Jacuzzis, and fruity milk bottles of contemporary Japan, he goes back to his ancient Rome and revolutionizes bathhouse design, gaining the favor of the emperor and citizenry in the process. But as Lucius goes back and forth between Rome and Japan, he gets further caught up in the life of suffering manga artist Mami (Aya Ueto), as well as his own emperor, who has become the victim of a widespread conspiracy to overthrow his rule. Filmed in part at Rome’s Cinecitta Studio and based on Mari Yamazaki’s popular manga series, Thermae Romae is a fun and funny dedication to the Japanese obsession with cleanliness, bolstered by a pitch-perfect comedic performance by Abe as the wet and naked Roman architect.

風呂を愛する二つの民族が出会ったとき、世界の歴史が大きく動き出す！
古代ローマ帝国の浴場設計技師ルシウス（阿部寛）は、生真面目すぎる性格が時代の変化に合わず、職場を解雇される。落ち込む彼は友人に誘われて訪れた公衆浴場で、突然現代日本の銭湯にタイムスリップしてしまう。そこで出会ったのは漫画家志望の真実（上戸彩）ら「平たい顔族（＝日本人）」。ルシウスは日本の風呂文化に驚愕、打ちのめされる。しかし、古代ローマと現代日本を行き来しながら日本の風呂文化をローマで再現していく彼は、たちまち評判の浴場技師となっていく。その評判を聞きつけた時の皇帝ハドリアヌス（市村正規）は、自分の後継者にと考えているケイオニウス（北村一輝）のためにルシウスに大浴場を作るよう命じる。次第にルシウス、そして真実までもがローマ帝国の運命に深く関わっていくことに……。