The monks of Shrewsbury Abbey seek the relics of a saint for their chapel, in Wales. The locals object to this translation of the relics, and a local leader is found murdered. Brother Cadfael is challenged to bring right endings to all parties, in Wales and in the Abbey.
In May 1137, Prior Robert of Shrewsbury Abbey is determined that the Abbey must have the relics of a saint. Finding no suitable local saint, Robert finds one in nearby Wales.
Brother Cadfael has two novices assisting him in his herb and vegetable gardens: John (practical, down-to-earth, whose vocation Cadfael doubts) and the ambitious Columbanus (of whose illness Cadfael is sceptical, although he treats him with sedating poppy syrup). Columbanus and Brother Jerome, Robert's clerk, go to Saint Winifred's Well in North Wales for a cure. When they return Columbanus says Saint Winifred appeared to him, saying that her grave at Gwytherin was neglected; she wished to lie somewhere more accessible to pilgrims. Abbot Heribert approves the trip to Wales to retrieve Winifred's remains. Robert, Sub-Prior Richard, Jerome and Columbanus are joined by Cadfael (fluent in Welsh) and John (for menial work).
The bishop of Bangor and Owain Gwynedd (prince of Gwynedd) consent. When the monks reach Gwytherin (on the Cledwen River) the local priest, Father Huw, objects to Winifred's remains being removed without approval by the free men of the parish. Rhisiart, the community's most influential landowner, opposes Winifred's removal. Robert tries to bribe him, and Rhisiart storms off. The assembly dissolves, agreeing with Rhisiart. Father Huw persuades Robert to ask Rhisiart for another meeting the next day, to which he agrees. While John helps the servants (and Jerome and Columbanus keep vigil at Winifred's chapel), Robert, Richard, Cadfael and Huw await Rhisiart; he does not appear. The landowner is found dead in the woods, with an arrow in his chest bearing the mark of Engelard (an Englishman in love with Rhisiart's daughter, Sioned). When Engelard appears, Robert insists he be taken into custody. Engelard flees, and Brother John impedes the only local man close enough to stop him. Robert orders John held for breaking the law of Gwynedd and his vow of obedience; this pleases John as he is held where his love, Annest, lives.
Cadfael realises that Engelard's arrow did not kill Rhisiart: his back is damp, while his front is dry. Rhisiart was stabbed from behind by a dagger, falling face-down. After it rained, someone turned him over and pushed an arrow into the wound from the front. The locals see Rhisiart's death as an omen, and agree to Winifred's removal; Robert plans to exhume her remains after a three-night vigil. Cadfael hopes the superstition that a corpse will bleed if touched by the murderer will force a confession. At his suggestion Sioned asks that after each night's prayer, those keeping watch place their hands on Rhisiart's heart. Jerome does so, but Robert refuses. The third night, Cadfael and Columbanus keep watch. Columbanus has another seizure; he is removed unconscious in the morning, evading Sioned's request. He recovers after mass, saying that Winifred told him Rhisiart should be buried in her grave when she is removed.
Winifred is exhumed, her linen-wrapped skeleton placed in the coffin brought from Shrewsbury and the coffin sealed with wax. As Rhisiart is prepared for burial, Sioned asks Peredur (another suitor) to place a jewelled cross on his body. Peredur refuses, confessing that he found Rhisiart dead and pushed Engelard's arrow into the wound so Engelard would disappear as rival for Sioned's hand. Cadfael finds the flask of poppy syrup (brought for Columbanus) nearly empty, recalling that when Rhisiart was murdered, only Jerome drank the wine provided for the vigil; if Jerome slept through the vigil, he would be ashamed to admit it. Before the monks depart, Columbanus offers to keep vigil and falls asleep; a vision of a young woman wakes him, asking why he murdered Rhisiart. Columbanus confesses, begging forgiveness. Touching her veil, Columbanus realises the saint is Sioned and slashes at her with a knife before fleeing. Cadfael and Engelard tackle him outside, and Engelard accidentally breaks Columbanus' neck. Cadfael acts quickly; he, Engelard and Sioned undress Columbanus, open Winifred's coffin, replace her above Rhisiart's body and place Columbanus's body in the coffin and ensure that the coffin appears undisturbed.
Columbanus' sandals, shirt and habit are found on the chapel floor, with hawthorn petals around them. Robert proclaims that Columbanus's prayers have been answered. The villagers load the saint's coffin on a cart, and as they leave Cadfael sees John bidding them farewell.
Two years later Bened calls at Shrewsbury, telling Cadfael that John and Annest are married and John will become smith after Bened. Sioned and Engelard, also married, have named their child Cadfael. Bened also notes that Winifred's former resting place is the scene of pilgrimages and cures; the Abbey reliquary is ignored by pilgrims. Cadfael muses that the saint will not mind sharing her grave with Rhisiart.