Alder has always lived in his cozy little house in Southern California. And for as long as he can remember, the old, reliable, comforting walnut tree has stood between his house and the one next door. That is, until a new family--with a particularly annoying girl his age named Oak--moves into the neighboring house and, without warning, cuts it down. As Oak and Alder start school together, they can't imagine ever becoming friends. But the two of them soon discover a series of connections between them: mysterious, possibly even magical puzzles they can't put together. At least not without each other's help.