Mouse is on the run, having escaped (via the aquaducts) from the allegedly inescapable Dungeons of the walled city Aquila. An attempt to recapture him, on the orders of the power-hungry, evil Bishop, is halted by the entrance of a mysterious knight on a war-horse bearing a hawk. The stranger, the ex-Captain of the Guard, Etienne Navarre, saves Phillipe and somewhat befriends him. As night approaches, Navarre and his hawk disappears and a beautiful woman and a dangerous black wolf appear out of nowhere.

Phillipe, who has taken a liking to Navarre, is somewhat smitten with the Lady Isabeau, but she is gone in the morning while Navarre has returned. As the days and nights pass, Phillipe befriends Isabeau and, being a romantic, delivers poetic messages to Navarre and Isabeau that were never given, saying that the other said these wonderful things during the night and the day, respectively. He learns that Navarre requires his knowledge of the tunnels and sewers to get into Aquila Castle, as Navarre intends to kill the evil Bishop of Aquila. Navarre says little, but his ruthlessness is persuasive, and Phillipe is quickly convinced to accompany him back to Aquila.

The hawk is shot with an arrow during a fight with the Bishop's guards and Phillipe takes it to a ruined abbey in which Father Imperius lives and there he learns that the Bishop of Aquila, who lusted after the lovely Isabeau, placed a devil-pact curse on Navarre and Isabeau when he found out that they were in love. By day, Navarre is human, but by night he is a large, black wolf. By night, Isabeau is human, but by day she is a hawk. Two years have passed since the spell was cast, and Navarre is determined to kill the Bishop and exact his revenge. Father Imperius has a plan, knowing that a time is fast approaching when it will be neither day nor night, and that they will be able to stand together as man and woman in front of the Evil Bishop and break the curse. But Navarre won’t listen. Mouse and Imperius must convince him otherwise, or the lovers will cursed forever.