Finn's just a regular high school junior - he plays baseball, has a funny best friend, and gets along mostly fine with his family. He's had epileptic seizures since he was 6 and was hurt in a freak accident that also killed his mother, but he deals with it pretty well.
Oh ... and his father wrote a famous sci-fi novel that pissed off a whole bunch of religious people. And it may have featured Finn (characteristic scar and all) as one of the destructive carnivorous beings from another dimension bent on taking over Earth.
Not a lot happens in this book - it's more character-driven and deals with Finn's asserting his independence, falling in love for the first time, and trying to figure out his place in the world (without eating people and taking over, like in Dad's book).
I have LOVED every Andrew Smith book, and this one's also wonderful. Perhaps it's not so over-the-top like Grasshopper Jungle, but a great read and worth the time none the less. It's funny and heartbreaking, there are moments of suspense and mistaken identity, but mostly it's simply about becoming Finn.