When an American monsignor with high-level political contacts is found murdered at the foot of Death's Door, one of the five entrances to Saint Peter's Basilica, Lieutenant Billy Boyle is put on the case. To solve this murder, Boyle first has to be smuggled into Rome, while avoiding the Gestapo and Allied bombs. Then he must navigate Vatican politics and personalities—some are pro-Allied, others pro-Nazi, and the rest steadfastly neutral—further complicated by the Vatican's tenuous status as neutral territory in German-occupied Rome. But Boyle's ready to risk it all because of one simple fact: Diane Seaton, his lover and a British spy, has gone missing while undercover in the Vatican. After he discovers that she's being held in the infamous Regina Coeli prison, just a short walk from the Vatican border, Boyle must decide whether he dares attempt a rescue, even though a failed effort would alert the Germans to his mission and risk an open violation of Vatican neutrality.