The series revolves around the adventures of the title character, Johnny Test. He is an 11 to 13-year-old suburban boy who lives with his super-genius 13 to 15-year-old twin sisters, Susan and Mary, both of whom are scientists. Johnny is often used as a test subject for his genius twin sisters' inventions and experiments. These range from gadgets to superpowers. Their experiments often cause problems that he must resolve, and he must sometimes fight villains in the process. He occasionally saves the world with his sisters' inventions. Johnny's dog Dukie has been given the ability to speak by his sisters.