Book #1 of the Papa Pontivy series.
The year is 1939. War is imminent in Europe but Captain Newman is on his honeymoon traveling the Continent. Papa Pontivy summons him to help investigate a "seemingly impossible murder" in one of the fortified bunkers making up the Maginot Line. They will have to deal with enemy spies even more than killers.
Marshall is mentioned a couple of times but plays no part.