Fourth-grader Scab McNally knows exactly what he wants: a dog. Part inventor, part daredevil, and part mischievous student, Scab is determined not to let anything or anyone get in his way, especially his "smart times ten" twin sister, Isabelle. With her reporting his every move to their parents, Scab can't catch a break. How is he supposed to prove to his parents that he's responsible enough to handle a pet if she keeps tattling on every tiny thing he does? When his newest invention, a sister repellent spray (a.k.a. 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? Soon, the money is flowing in. But one broken bottle of the mega-stinky concoction brings a brownish-gray cloud of disaster, not to mention a Haz Mat crew. And faster than a bunch of girls in Hello Kitty shirts can scream "Ewww!" Scab's hopes of getting a dog vanish (cough!).

In the midst of his self pity, Scab begins to see that his at-any-cost drive to get a dog has hurt the people he cares most about. Will Scab be able to set things right?