Jack might be the only kid in the world who is dreading the summer. But he's got a good reason: summer is when his single mom takes a second job and leaves him at home to watch his autistic kid sister, Maddy. It's a lot of responsibility, and it's boring, too, because Maddy doesn't talk. Ever. But then, one day at the flea market, Maddy does talk - and tells Jack to trade their mom's car for a box of mysterious seeds.