Lots of girls play Fairy Princess when they're little. Megan O'Reilly had no idea the real thing was like playing chess, guitar, and hockey all at once. Megan had known for a long time that she wasn't an entirely typical girl. But living with ADHD—and her mother's obsessions—was a very different thing from finding out she wasn't entirely human. Somewhere out there, in a completely different world, her father needs help. There's a conflict, revolving around Faerie seasonal rituals, that could have consequences for humanity—and if Megan's getting the terminology straight, it sounds like her family aren't even supposed to be the good guys. As she's further and further swept up in trying to save her father, Megan may be getting too good at not being human.

School is out for the summer, which will give Megan O'Reilly's friends time to see that she really starts learning something useful. When music is magic and magic is sometimes medicine, it's easy for a menehune BFF, a satyr rock star, and a disabled pixie with a service crow to give a girl information overload. The weekend of the Fremont Solstice Fair is also the weekend of the Goblin Market. Megan's adjusted enough to this Unseelie Princess thing to see some use in all this colorful shopping potential, both real-world and magical, even if she can get ahead of herself sometimes. But someone wants to use the Market for his own destructive purposes. And he knows a guy who's great at ruining other people's Midsummer nights.

The faeries are preparing for a war throughout worlds. But before the fighting comes the talking. May is approaching, but there's no way Megan O'Reilly can focus on plans for graduation and beyond when the ancient monsters called the Fomoire are about to break out of their frozen lake. As the ice melts in the Spring air, a conference of magical beings from around the world is being called. Apparently there are a lot of risks involved in properly equipping this kind of party. There are so many places—Earth, different mystic realms, the collective unconscious, an outright netherworld—to go, people to see, and things to be done. Megan is going to help. No matter how dangerous it gets.

Late December is a magical time of year. Magical enough for the forces of the Fomoire to truly begin their conquest. Many people travel and consider new things during a gap year before college. Megan of the Unseelie, all of 18 now, is spending hers hurrying between realms and trying to figure how to stop the Fomoire from running rampant over all worlds when the Winter Solstice arrives. Now Megan must rally her friends and allies one more time in a last, desperate mission. Especially because some on both sides say all those worlds are doomed.