What Amazon has to say:

Harlequin is the first book in Cornwell's Grail Quest series, which chronicles the adventures of young Thomas of Hookton, "a big, bony, black-haired country boy". Thomas rejects the church in favour of the life of an archer in France after his village is brutally sacked by the French. The young Thomas fights back against the French with his bow, and "in that one instant, as the first arrow slid into the sky, he knew he wanted nothing more from life". He vows to seek revenge on the plains of France, and recover the holy relic of St. George stolen from his village by the sinister "harlequin" with whose destiny Thomas finds himself inextricably entwined. The rest of the action moves at a hectic pace across the violent and bloody battlefields of northern France, as Thomas falls for a beautiful French widow nicknamed "the Blackbird", makes a mortal enemy of the "poor, bitter and ambitious" Sir Simon Jekyll, and follows the ensign of King Edward III and his heroic son, the Black Prince.