The narrative begins with an introduction to the pervasive influence of the supernatural in Irish history and folklore. The story is presented as a translation from the Gaelic memoirs of Captain Turlogh Kirowan, an Irishman who served in the French army during the 17th century. The events described take place during Cromwell's brutal campaign in Ireland.

Captain Turlogh Kirowan, known for his valor and defiance against English rule, finds himself alone and hunted in County Clare after the collapse of Irish resistance in Munster. Seeking refuge for the night, he stumbles upon the ruins of a once-grand castle, formerly a stronghold of the O'Brien clan. The castle, now a desolate heap of masonry, symbolizes the fallen state of Ireland.

Exhausted, Kirowan falls asleep in the great hall of the castle. He is abruptly awakened by a figure standing in the doorway, illuminated by the moonlight. The apparition is a tall, broad-shouldered man dressed in the attire of an Irish chieftain from five hundred years ago, holding a broken sword and clutching a bleeding wound on his chest. The ghost warns Kirowan in Gaelic, "Take care! The red-beard is coming!" before collapsing and disappearing without a sound.

Confused and disturbed, Kirowan initially believes he has had a vivid dream. However, recalling the ghost's warning, he decides to leave the castle and hides among some bushes outside. Soon, a group of Roundhead soldiers, led by Captain Balston—a large man with a red beard—approaches and surrounds the castle. Realizing the ghost's warning was real, Kirowan sneaks away, finds a tethered horse, and escapes to Galway, eventually reaching safety in France.

In France, Kirowan recounts his tale to his family and their friend, Lady Nuala O'Brien. Lady Nuala identifies the ghost as Conmac O'Sullivan, a chief from Kirowan's mother's lineage. She explains that centuries ago, Conmac had warned the O'Briens of an impending Danish raid with the same words he used to warn Kirowan. Thanks to Conmac's warning, the O'Briens had repelled the Danish attack, led by a man known as Lamh Derg, or Red Beard. Lady Nuala asserts that the apparition of Conmac O'Sullivan had saved Kirowan from capture, reaffirming the deep connections between family, history, and the supernatural in Irish culture.