This book is a series of short stories of the hero Fionn MacCumhael. It tells of Fionn being descended from the Winter Queen that first came upon the islands. It tells briefly of his early life and several adventures he had.

Shortly before Fionn was born his father, Cumhal, was the leader of the Fianna warriors. His mother, Muireann Muncháem, was the daughter of a nature speaker, or druid, called Tadg mac Nuadat. The druid foresaw her marriage and that would ultimately lead to the loss of his ancestral seat. Tadg didn’t hide the fact he disapproved of his daughter’s relationship. Cumhal, who didn’t honor Tadg’s wishes, kidnapped Muireann so they could live a free life.

Outraged at what had happened to his daughter, Tadg appealed to Conn of the Hundred Battles for assistance. Conn also forbid the relationship so he set out after them. During the Battle of Cnucha, Cumhal was slain by Goll mac Morna who then became the leader of the Fianna.

Later Tadg rejected his daughter Muireann who was now pregnant.  Tadg prompted his followers to shun and berate her but Conn disagreed so he intervened. Conn sent Muireann to live with his sister, druidess Bodhmall, and her husband Fiacal mac Conchinn.

Muireann gave birth to a son named him Demne. For his own protection he was called Fionn and left under the care of druidess Bodhmall and a female warrior called Liath Luachra. Both women raised him in secret, in the forest of Sliabh Bladhma. They taught him all the knowledge they had, including how to be a warrior.

As word began to spread of a young warrior called Fionn his father’s enemies were sure to find him. With all his knowledge both his foster mothers sent him on his way to survive on his own.

When Fionn was a boy he was apprenticed to the druid FInegas who was obsessed with catching and eating the Salmon of Wisdom. Eventually he did catch it, and ordered Fionn to cook it. Unfortunately, while he did so, Fionn was splashed with a little hot oil from the fish and put his thumb in his mouth to soothe the burn. Thus, he was the first to consume any of the Salmon of Wisdom. This made him very wise, and very knowledgeable.

When he was still young Fionn heard of the terrible goblin, Aillen Mac Midgna who would put people to sleep with the playing of his harp. Aillen would then burn the people and their halls and homes alike. This is a trick he particularly liked to play on Samhain at the Hill of Kings. With all his knowledge, Fionn was able to brew himself a tincture that made him immune to Aillen’s music. He was then able to kill him with a great spear, deadly to goblin-kind. After this victory, Fionn was made leader of the Fianna. Even Gol Mac Morna, it’s former leader and enemy of Fionn then swore to follow him.

When Fionn came of age his people were at war with the twisted and cruel Fomóiri. The stories tell of the Fomóiri being misshapen in body and thought, wielding dark magick. They were also enormous creatures standing as tall as two Sidhe in many cases.

Fionn fought against them many times. Especially their cruel leader Balor One-Eye. Fionn lead The Fianna very successfully over many years. Perhaps decades or centuries.

Some of the stories tell of Fionn being able to grow to enormous size so as to better fight his giant adversaries. Fionn is also said to have mastered many arcane martial powers of various sorts. He drove Balor and the Fomóiri off the islands. He also fought the last vestiges of some Firbohlg forces that still lived on the islands since before The Sidhe arrived. It is also said that he had a sense for when and where the Ekvotar may strike, and beat them back many times, even as they tried to find ways to reach Tir-na-Nog through the gateways near the Hill of Kings.

Fionn came upon a Sidhe settlement one day after a fight with a band of Firbohlg. He and The Fianna sought rest and revelry after their great fight. But, as he arrived the people of the settlement told Fionn that three of their children had been taken. Fionn raced into the woods and found traces of passage. He followed the tracks and eventually found a war band of Ekvotar. The Fianna engaged them in battle, but some retreated to a  cave nearby. Fionn followed and found them cutting the children and gathering the blood into a cauldron glowing with energy, clearly very important to the foul Ekvotar. Fionn drove off the Ekvotar and then smashed the cauldron. The three children were saved and ran back to their home.

Ab’dusha, one of the Fianna, took the broken pieces of the cauldron and returned with it to Tir-na-Nog. There she forged it into a set of armour and gifted it to Fionn to wear in subsequent battles.

One day, when Fionn was leader of the Fianna, a deer came into their encampment. Having done so, the deer transformed into a beautiful woman, Sadhbh. She had been cursed by a cruel magician to be a deer. But, the magic did not hold in the Fianna camp. Fionn fell in love with her and she with him. They eventually had a son, Oisin. However, one day when The Fianna were off hunting, she was lured from her home by a vision of Fionn. Then the cruel Fear Doirche turned her into a deer again. She fled into the woods. Fionn spent many years searching for her, but was never able to find her again. Fear Doirche, it is said, eventually caught Sadhbh using the magic of the Children of Danu he had learned and corrupted. He then took her to gift to Balor One-Eye, so the dread Fomóiri could cook and eat the deer that was their greatest enemy’s beloved wife.

At one time, Fionn built himself a bridge of stone and magic to reach the land of mountains and lakes. At that time it was ruled by a fierce Fomóire named Benandonner. He ruled from the mountain of storms, far from the shores. Fionn lead the Fianna on many raids leading further and further inland. Eventually they encountered Benandonner and the fight went poorly, the Fianna were forced to flee back across the bridge to the isle of emerald forests. When Benandonner eventually followed, Fionn used trickery, and the clever words of his wife, Sadhbh to convince Benandonner he could not defeat Fionn. This gave Fionn time to rest and to grow to enormous size and engage Benandonner in battle again. Benandonner was driven back, and after such a defeat he posed little threat to the Sidhe thereafter. 

Strangely, however, Fionn did not go to war with humans on the isles when they started to arrive. That is, aside from some specific skirmishes with wrongdoers or those encroaching on Sidhe lands. Rather, he seemed to prefer to let the humans settle in small defined areas so long as The Sidhe and Fianna had the run of the islands. This put him in some conflict with other leaders of the Sidhe who favoured driving the humans back to the sea, as they had done with the Fomóiri and the Firbohlg.