According to IMDb and some of the other reviewers, this episode of "Cadfael" is not really based on Edith Pargeter's novel of the same name. Unlike her other stories in the series, the BBC did NOT try to stick close to the original source material but kept just the name of her story.
The episode begins with a bunch of pilgrims descending on the abbey--seeking relief from their various leg ailments. However, a weird thing occurs--the monks discover a dead guy in a sack among the pilgrims' belongings! Cadfael carries out a bizarre sort of autopsy by boiling away the bones from the corpse and sees that the man appears to have died from a blow to the head--and it's assumed it is murder. Who was the dead man? Why was he brought along on the pilgrimage? And, most importantly, if there was a murder, who did it? It's all up to Cadfael to figure it out--but there aren't a lot of clues, though the film seems to indicate that some brothers might have something to do with this.
The solution to all this, though interesting, is VERY familiar to mystery films. The guy who THINKS that he murdered the man didn't actually do it--and another used this as a chance to kill the victim and blame it on the first guy. Very familiar...too familiar. A weak final episode, I assume just filming the book as it was written would have been better.