Agatha Cackle
A Bolt from the Blue is episode 13 of the first season of The Worst Witch TV series. It was Preceded by Sweet Talking Guys, and followed by Old Hats and New Brooms.
Youtube links: part 1 part 2 part 3
Plot
It is nearly the end of the year for the student witches, and Mildred's incantations still don't seem to work as well as intended. To cure a case of hay fever brought on by one of Mildred's errors, the other teachers give Miss Cackle what they assume to be a remedy potion, but which is in fact a mislabeled experiment of Miss Hardbroom's. The other teachers make no mention of the mishap, even when Miss Cackle's behaviour takes a turn for the malicious, especially towards Mildred. She announces to the staff that she is going to expel her, which is a shocking fate, even for the worst witch. It is Mildred and company, however, who realise that it is not Miss Cackle, but her evil twin sister Agatha back to cause more trouble.
The girls track down the real Miss Cackle to the store cupboard in which Mildred and Enid were once trapped. Unfortunately, they are discovered by Agatha's two wicked cronies, Coldstone and Bindweed, and time is ticking away to midday on the last day of term. The final confrontation takes place in front of everybody and Mildred saves the day, for which she earns a raise in her final grade from C-- to A+.
Quotes
Davina: "She's got a touch of hayfever!" Constance: "What have you given her...ah....I see...yessss....could I have a word.....DAVINA! This potion isn't quiteb what it says...I was just recycling the bottle." Davina: "Highly commendable of you Constance! but what will it do to her?” Constance: “how would I know, it was just an experiment!” (in the background, Miss Drill is Giving Miss Cackle the water from the flower vase, and then the water from the fire bucket)
Miss Cackle: “I’m not Agatha, I’m Amelia, and I’M the headmistress HERE!”