Gladys of Harlech was first published anonymously in 1858. The novel is set during the Wars of the Roses in the mid-15th century and tells the story of Gladys, granddaughter of the last Welsh keeper of Harlech, and her family.

Fighting on the side of the House of Lancaster, Gladys and her family flee into hiding in the mountains after Harlech Castle falls into the hands of the Yorkists at the end of a long siege.

Years later, the new tyrant steward of Harlech, Sir Gilbert Stacey, captures Gladys and presses her into serfdom, not knowing he has caught the heiress of the castle. During her arrest, Gladys falls in love with young Ethelred Conyers, but finds herself trapped between her sense of duty for her oppressed people and her affection for the young English nobleman.

After escaping from Harlech , Gladys is sent on a quest to France by the Dewiness, a soothsaying witch. The Dewiness hopes the beautiful Welsh princess will lure Henry Tudor out of his French exile to free England and Wales from the clutches of King Richard III. Switching sides and joining the cause of the Red Rose, Ethelred follows Gladys on her dangerous journey.