Only child Kimberly MacMillan, 24, inherits her mother’s house when Mom passes away. She finds the house is full of forgotten memories, dating back to when she was six and first moved in with her parents.
In her bedroom closet appears a multicolored unicorn, named Boo, from the land of Ghiliop. He and all his land’s residents need her help with their “tethering” to ensure their survival. She goes to Ghiliop with Boo – meeting Again, the echo fairy; gnome scientists Better, Late, and Than Never; and giants named Now and Later.
After many suggestions and much thinking, she stumbles on the solution that will keep the Ghiliopians tethered and safe for years to come.
Adult Kim is haunted by her six-year-old self. Boo has come back and needs her help again. Kim has forgotten about him because of the need to focus on her grown-up world. She is able to help the Ghiliopians with their tethering but must say goodbye to Boo a second time.
Later, Kim gets the idea to abandon California and live in the house with her boyfriend. If Boo and his friends ever need her help again, they'll know where to find her.