Mysterious murders have been taking place in the remote villages of the great swampland. A savage wolf-like but apparently human killer has been seen briefly by terrified witnesses to the crimes. This killer seems to disappear into the air after the murders. Rumor among the fishing people says that the werewolf is a master actor who can look like anyone - possibly he is an ancient one returned to take revenge on the newcomers who have ravaged the land - they call him Zakotu, the Spirit of the Swamps. The play begins as the mother of the local priest, Matilde, is about to usher in an old friend. The friend is Doloree, a nun and nurse. Or is she? Indeed, is anyone what they are supposed to be? Matilde is murdered. So, whodunit?