When a mosquito bite transfers Ozzy and Drix into the body of a boy, Hector Cruz, another germ comes with them. It is up to Ozzy and Drix to capture this germ, Scarlet Fever, before it has disastrous consequences.
Ozzy crashes his car into a nerve causing Hector's leg to fly out. Normally that wouldn't be much of a problem, but this time Hector accidentally tripped the local school bully, getting him into a fight after school.
As Hector finds a stray dog outside, an allergen is wrecking Hector's insides. The size changing allergen alternates between a huge monster and a meek guy. At the same time Drix finds a pet dog cell of his own.