Horrid Henry and the Evil Mastermind is episode 49 of Series 4.
In school, while Miss Battle Axe is teaching she askes Linda what is the answer. Linda still sleeping, replies the answer which is correct and Miss Battle Axe askes anyone who listened and Brain answers with another answer and Andrew gives an answer as well. When the bell goes, Linda, Brian and Andrew stay in with Miss Battle Axe who turns her back and evilly laughs.
After break, Miss Battle Axe comes in but with a song. And shows that Linda, Andrew and Brian have become her minion. Her attitude has strangely changed and Henry goes to find out what's going on. Margaret thinks she was taken over by an evil mastermind but Henry disbelieves that but Ralph says that she may be right.
Al plans an idea that to think with cleverest people they know and then ask them if they know anything about the mastermind but they know they'll be onto them. At home, Peter is called to be known who much he knows but he is confused. Henry asks Peter where he is going. Peter says he has a meeting with the Best Boys.
Henry is suspicious and tells Ralph who thinks that they are normal. Henry asks Peter if he is an evil mastermind but Peter does not think he is. At school, Margaret asks Henry if Peter was the mastermind but Henry says it wasn't him. Miss Battle Axe comes in and Margaret ask if she was clever but when Miss Battle Axe's back was turned, Margaret places a tracking device in her pocket.
Miss Battle Axe leaves the room and Margaret sees on her phone where she is going. Miss Battle Axe is in Miss Oddbod's office and soon Henry thinks she was the mastermind. They set a rope signal and Miss Oddbod comes in. Margaret says to Miss Oddbod they're onto her. But Henry hears noises from the office.
Henry finds out that Miss Oddbod was not the mastermind and Miss Oddbod opens the office. Miss Battle Axe is in there calling someone called Big Big Sister. Miss Battle Axe replies that she was talking through the computer but Miss Oddbod says that is not possible and takes Miss Battle Axe to the teacher's lounge.
Henry and Margaret argue about who's clever or not while the computer receives an eerie voice message before ending the episode hearing.
'Calling Big Sister... Calling Big Sister... Big Sister, are you there?...'
- Miss Oddbod says it's not possible to talk through the computer but if Miss Battle Axe was using an app similar to Skype then she may not heard about it.