Johnny Test is a troublesome 11-year-old who dreams of having superpowers. Luckily, his twin sisters are scientists, so he has plenty of chance to mess around with their experiments. Johnny spends most of his time with his best friend, a talking dog named Dukey.
Johnny Test's pet Dukey is the only talking dog in Porkbelly. Susan and Mary genetically mutated him (also giving him great kung fu skills), and now he winds up getting dragged into Johnny's silly schemes. He usually disguises himself as a "kid with a rare hair disorder".
Johnny Test's genius twin sisters Susan and Mary frequently use him as a test subject for inventions, most of which are to impress their good-looking neighbour, Gil. They are the only 13-year-olds clever enough to attend the Porkbelly Mega Institute of Technology.