It is February 1194. A desperately ill man is making for Hawkenlye Abbey in the hope of a miracle cure. In his delirium he sees the Virgin Mary and, sinking to his knees, he begins to pray. She is the last person he will ever see.

The winter cold intensifies and the Vale lake freezes over. It is only when the thaw sets in that a corpse is discovered in the icy waters, its skull crushed by a lethal blow. With no clues on the body but an apothecary's remedy, Abbess Helewise asks her trusted friend Sir Josse d'Acquin to find out the man's identity. As Josse sets out on his mission, a party of sick people arrive seeking help, and their sickness looks terrifyingly like plague . . .