CITADEL

Raiders of the Lost Ark meets The Da Vinci Code, with lashings of Nazis and belles mademoiselles.

Yes, it’s improbable in the extreme that a medieval codex should figure high on the list of priorities of both the Gestapo and the French Resistance, but, well, the Nazis were an improbable bunch, and they actually had a noted medievalist on their payroll against the odds of turning up the Holy Grail or other mysteries of the ages. Improbability doesn’t get in the way of Mosse’s (Sepulchre, 2008, etc.) yarn, which, though very long, is full of rousing action and intelligent character development alike. Closing her Languedoc Trilogy, she turns in a tale that begins, gruesomely, with a retaliatory hanging and moves swiftly to a firefight and a grimly delivered piece of partisan justice—and that’s within the space of just a few pages. Interwoven in the tale of the doings of a girl gang of Resistance fighters in Vichy, France, code-named Citadel, are spectral events from another time, about which a curious fellow named Audric Baillard seems to know altogether too much. Tough-as-nails Marianne Vidal is one of the fiercest of the fighters; her sister Sandrine joins her as soon as she’s old enough to get a driver’s license. The sisters are of an ancient clan (“ ‘Names are important,’ Baillard said brightly”), and both are attuned to the things that go bump in the night. But can both outlast the SS thugs who are tearing around Carcassonne? Mosse slips a millennium and a half and more into the past to introduce an ancient heretical document, the Languedoc being a place notably receptive to heretical ideas, the discovery and mastery of which will allow its holder to conjure up an ancient ghost army (“You want them safely in the earth, don’t you, Audric?”)—not at all a bad thing to have if you’re out for world conquest, that. The bad guys are bad, a local collaborationist particularly so; the ghouls are ghastly; the Nazis, determinedly Teutonic; and the filles de France, fetching.

Suspend disbelief and enjoy the time travel and genre-blending.