The series follows Dr. Kenzo Tenma as he pursues a young psychopath/sociopath named Johan, whose life Tenma once saved. The story rapidly progresses through a number of locations: it starts in Düsseldorf, Germany, passes through Berlin, Frankfurt, Munich, Wiesbaden, cities in the Czech Republic such as Prague, and other cities and villages.
Both anime and manga begin with a passage from The Revelation of St. John the Divine, Chapter 13: Verses 1 & 4
And I saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads, the names of blasphemy. And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, "Who is like unto the beast? Who is able to make war with him?"
The quote is often interpreted as the coming of the Antichrist. This quote is used because of the many parallels and comparisons between one of the main characters of Monster (Johan) and two candidates of the Antichrist as predicted by Nostradamus, the two were commonly Napoleon Bonaparte and Adolf Hitler.