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!
Thermae Romae takes anime and turns it upside-down!