If you're interested in an audio edition, check that you're getting the unabridged recording narrated by Stephen Thorne.
Ideally, read all the preceding books in the series, in order, before reading this one. At a minimum, first read #1 (A Morbid Taste for Bones, the story of how the monastery came to have St. Winifred as its patroness) and The Virgin in the Ice, to avoid the biggest spoilers.
This June of 1141, the feast of the translation of St. Winifred dawns upon a time when the civil war between the Empress Maud and King Stephen for the throne of England may finally draw to a close: Stephen was captured at the battle of Lincoln, and even now Maud is negotiating with the city of London for her entry into Westminster for her coronation. The papal legate, Bishop Henry of Blois, brother to Stephen, has called a legatine council (including Abbot Radulfus from Shrewsbury) and is working on turning his allegiance to the empress, for the sake of peace. Hugh, sheriff of Shropshire for Stephen, broods on ways and means of getting a man into Bristol to free Stephen, and prays for a miracle, while using his friend Brother Cadfael as a sounding board.
Cadfael, too, is praying for a miracle - any miracle - at this feast of St. Winifred. Not from a desire for the abbey's glory, or from any faltering of his own faith, but as a sign that the saint took no offense from the events of _A Morbid Taste for Bones_, when he accompanied a delegation from the abbey to the saint's grave in Wales to bring back her mortal remains as holy relics. (Since that was before Hugh's arrival in Shrewsbury, Cadfael summarizes the story for him, so it's possible to follow the plot of _Pilgrim_ without reading _Bones_. But be warned that Cadfael reveals the ending of _Bones_ to Hugh.)
Abbot Radulfus returns in time for the festival, bearing word of a cowardly murder at the legatine council. The attempted murder of the envoy of Stephen's queen failed, but Ranulf Bossard, the brave man of the empress' party who foiled the attempt, was himself cut down in the street.
All the brothers are busily preparing for the huge influx of pilgrims at this time of year, many of whom are ill and seeking miraculous healing. Brother Cadfael, as herbalist, sees some of the more noteworthy cases: Rhun, a devout half-Welsh boy with a twisted leg that might respond to treatment; his sister, Melangell; a young Welsh clark, Ciaran, traveling barefoot and wearing a large iron cross, on his way to Wales to die; Matthew, Ciaran's faithful shadow. There are less savory characters, as well, petty (and not so petty) career criminals who prey on the credulous and the frail. (Credulous, as in, people who trust a stranger's dice.) Some may even have fled from a city too hot to hold them.
Into this festival atmosphere rides a young envoy of the empress' party, on a twofold mission: to sound out Hugh on the question of his fealty, and to seek Bossard's young heir, who disappeared in this direction after his lord's death. But even if he is among the pilgrims, how can he be identified by those who have never seen him? And was he involved in Bossard's death?