Meet Scab McNally, a fourth grader with a mission. Part inventor, part daredevil, Scab is determined to let nothing get in the way of convincing his parents to let him buy a dog -- especially his "smart times ten" twin sister, Isabelle.

When his newest invention, a sister-repellent spray (aka Isabelle's smell) becomes a hit with the guys, Scab hatches a plan. He'll earn enough selling Isabelle's smell to buy a dog, and then his parents will have to say yes, right? But one broken bottle of the megastinky concoction brings major disaster, not to mention a hazmat crew. Will Scab be able to set things right?