Aslyn's Unicorn is a story of a young girl named Aslyn who is the pride of her father, King Stefan. At the start of the book, she is a precocious child just turning twelve, who, on her daily walks, encounters an adult white horse with a glowing, golden horn in the center of its forehead. Talking with her father, King Stefan, she learns that the creature she saw was a unicorn and that they are rare, beautiful Faerie creatures. Thinking and dreaming of the unicorns convinces Aslyn that she should go on a quest like her mother had before her and try to claim one of the mystical creatures for herself.

The quest starts with Aslyn traveling into the forest in her father's kingdom, and she uses what her daddy taught her about living outdoors. Her travels are uneventful until she goes to a small village for a tavern meal. There, she is accosted by a slovenly man in the tavern who mistakes her for a serving wench. Aslyn is saved by the village priest, who takes her home and feeds her and lets her sleep in his chapel house. The following morning, Aslyn meets the other occupant of the chapel house, a girl only a little younger than she, named Mirrile. She has purple eyes and wisdom beyond her years. The two girls get close quickly and share secrets of their lives, and Aslyn learns from the priest and Mirrile that there is a Land of Faerie and that the creatures she quests for are alive beyond her father's kingdom.

The final part of book 1 finds Aslyn gazing upward to the peak of her world's natural wonder, the Worldfall, which plummets several thousand feet into a deep pool. Beside her are Aslyn's companions who had come to her from the start of her quest: Shadow, a 700-pound talking black wolf, and her mare from home, who she finds out can also talk, named Nightlight. The journey to the top of the Worldfall takes three days, and Aslyn's curiosity grows, as Shadow tells her he is taking her to someone called "The Lady" who rules the Land of Faerie. Shadow explains that the unicorns live at the top of the Worldfall with the other creatures of Faerie and that Aslyn's quest will finally be over but that The Lady wanted to speak with her about other matters.

Aslyn and Shadow finally arrive at the top and are met by the first of the Land of Faeries inhabitants a small creature known as a Bee Sprite, who leads them to the great oak, which is the home of The Lady and her court. Aslyn starts to learn about the Land of Faerie, including some of its dangers. But through getting to know the creatures, she finds that they are just like her own people and that the bad feelings that exist between the Land of Faerie and the land of the man-folk have become a disruption in her world. During her time, in the Land of Faerie, Aslyn learns that The Lady is her mother who is named Krystionna. She had disappeared when Aslyn was a little girl.

Aslyn finally learns why she had disappeared. She also learns that she has a sister, the young girl Mirrile from the village below. With the sisters united in front of their mother, The Lady gives the girls a new direction for their lives as teens. She wants them to work together to heal the split between the two peoples and reunite the world and its creatures.

The girls talk about it and decide to accept their mother's wishes, so as the seasons change in the lands below, the girls set off again in the company of Shadow, for Mirrile's village. Journeying back to the chapel house, Aslyn spends a day or so with the priest and Mirrile and then prepares to return home, a more mature young woman with a different outlook on the world around her. Saying good-bye to all her companions, including Shadow who returns to the Land of Faerie, Aslyn and Nightlight thread their way back toward the castle by the sea, in her home of Avalonia.