Billy, later William, is the protagonist, although in an interesting twist of the plot he turns out not to be the major figure in Mercury's schemes; he is, in fact, secondary. Normally well behaved, Billy opens the novel with an act of defiance when an aspect of his spiritual life is threatened—when Sister Symphorosa tries to ruin Christmas for her class by telling them that Santa Claus is a myth and that believing in him is a sin. Billy needs his spiritual life; his everyday life is unpleasant, lonely, and even frightful.

When Mercury offers him the power of the caduceus, he accepts it in the hope of avenging those who abuse him.

From first to last, he does not seem to fully understand one of the most important aspects of his power: That he cannot control it. He first creates a curse to hurt one...