You will find : Letters, photos, a submarine model, instruments of torture, weapons (normal and hidden in shoes), costumes (movie and historical), propaganda posters, paper clippings, film extracts, bombs and pretty much any other spy paraphernalia you could dream of. There are interactive games (a rarity in French museums!) an Enigma machine and - my favourite! - a roll of toilet paper with Hitler's face on it.
The exhibition is divided in two parts, which both makes sense (it's a large exhibition!) and allows for in-depth treatment of the exhibits. It is a bit dark and exceedingly crowded (but then attending an exhibition on a Sunday afternoon during its final weekends is a rookie mistake I normally take care to avoid) yet I found this detailed and well explained.
No wonder it was so popular.