The boy Tip has a secret.
Well, that's not quite true. He doesn't have the secret but there's a secret out there, that somebody else has, which concerns him. Normally, you'd say that he has a secret, but since he's unaware of the existence of said secret (of which he is a key component), you can't. Not really.
You could say that the secret is an intrinsic quality of his being and that, in a way, he carries it around with him wherever he goes, and therefor he has the secret. But, that's just philosophical handwaving. He doesn't know, so he doesn't have it. So there.
But it's there. The secret, I mean. And Tip's also there. Running away.
From a witch named Mombi.
Along the way he meets the Strawman, Tin Woodman, Jack Pumpkinhead, Sawhorse, Glinda, and others, before finally changing the land of Oz for good.