Regan loves her sister Luna, even though Luna never reveals herself in the light of day, even though Regan can't tell anyone that Luna even exists. And Luna does exist, although their parents would like to ignore her existence. The world isn't ready for Luna, because Luna is also Regan's brother Liam.
As a groundbreaking novel about teenage transgender identity, this is a pretty original piece of work. And once again, Peters hits on a pretty good story idea and does a great job developing her characters to rise to the occasion. But once again, I find myself frustrated. Define "Normal" was a great book. So was Keeping You a Secret. But they always fall a bit flat. This time it is the sheer annoyingly selfishness of Luna which comes off as a barrier more than a character flaw. It's really hard to sympathize with so much co-dependency floating around in the story. A shame, though, because this is otherwise a very good book.