Mr Nerdon is a teacher who appeared in "Horrid Henry's New Teacher". In the book, he was Henry's teacher at the start of the school year before Miss Battle-Axe. In the TV series, he was the substitute teacher filling in for Miss Battle-Axe while she had flu. Mr Nerdon is known to be the toughest, meanest, nastiest baddest teacher in Henry's school and is very strict. He takes no nonsense from anyone and it seems that he is undefeatable, until Henry has a brilliant, spectacular idea which resulted in Mr Nerdon fainting and being taken away from the school in an ambulance, and running away from the school in the book and TV versions respectively.