A Bad Spell for the Worst Witch is the third novel in The Worst Witch children's book series by Jill Murphy. It was preceded by The Worst Witch Strikes Again and was succeeded by The Worst Witch All at Sea. It features Ethel Hallow's little sister, Sybil Hallow, and her friend Clarice Crow.
Mildred Hubble returns to Miss Cackle's Academy for Witches for her second year, determined to lose her embarrassing reputation as "the worst witch in the school". After Maud Moonshine and Enid Nightshade arrive, the three bump into two first-years (one of whom reminds Mildred very strongly of someone, the other with ginger frizzy hair in bunches). The one who seems familiar bursts into tears and clings onto Mildred when she hears that a lot of the teachers (including Miss Hardbroom) are quite strict. Mildred also accidentally slips a made-up story about Miss Hardbroom turning a student into a frog when she was just two seconds late for a lesson. To the girl's horror, they also learn that Miss Hardbroom will be replacing Miss Gimlett as their form-mistress for their second year.
The next morning, a fire bell rings. Mildred scrambles into her clothes but notices outisde her window Miss Hardbroom standing in the playground surrounded by hazy purple smoke. Thinking she is in a state of shock, Mildred tries to douse her in freezing cold water but the bucket slips off her broomstick and lands with a clang on Miss Hardbroom's head. After witnessing the bucket incident between Mildred and Miss Hardbroom, Ethel Hallow confronts Mildred at lunchtime saying that the girl she told the frog story to was her younger sister, Sybil Hallow. After prodding at Mildred, Mildred loses her temper and insults Ethel's family ("All you Hallows are weeds, weeds, weeds!").
After an unfortunate flying lesson, where Tabby makes another terrifying attempt to avoid sitting on the end of the broomstick, Miss Hardbroom sends Mildred to her room to calm down. When sitting in her bed, however, Mildred accidentally falls asleep and doesn't hear Ethel creeping into her room. Mildred is awoken by her bedroom door shutting only to find that everything looks giant-sized, even Tabby. She peeks in the mirror only to find that somebody has turned her into a frog. Whe jumping onto her bed, the frog-Mildred notices a tatty handful of weeds on her pillow, no doubt left by Ethel as a reference to her family's insult and why she turned her into the frog.
Panicking, Mildred squeezes under the gap under her bedroom door and hops off to a potions lesson where everyone has noticed that she has disappeared. Miss Hardbroom, however, discovers her and puts her in a jar. During the potions lesson, Mildred manages to escape from the jar and hops over to Ethel's desk where she drinks an invisibility potion that turns her invisible. Taking advantage of her invisibility, the frog-Mildred hops out of the academy and lands in the lilypond on the castle grounds.
In the lilypond, Mildred meets another frog who also was a human once called Algernon Rowan-Webb. Algernon reveals that he was turned into a frog by one of his fellow magicians after the two had an argument. Algernon also reveals that the one thing he is craving is some toats and crumpets with butter.
Promising to come back for him, Mildred hops back up to the academy where Maud and Enid instantly recognise her. They take her to Ethel where she reluctantly turns her back into a human. Miss Hardbroom comes by and Ethel tells her that she caught Maud, Enid, and Mildred sneaking around the school. Miss Hardbroom is just about to give Mildred a punishment, when she notices that Mildred's feet are still invisible. Mildred tells Miss Hardbroom what really happened. Miss Hardbroom tells Mildred and Ethel to go to her office the next day.
In the morning, Mildred and Ethel have a brief squabble about whose fault it was until Miss Hardbroom turns up. After the full story is explained, Miss Hardbroom bans both of them from attending the Halloween ceremonies (which was the only chance Mildred would have to come into contact with a magician who could turn Algernon back into a human). With no other options, Mildred feels she has no choice but to kidnap somebody who is going to the Halloween festival. She tricks a third-year witch, Griselda Blackwood, into looking under her bed for an imaginary beetle where Mildred ties her up and gags her and takes her cat.
Later, Ethel discovers Griselda being held captive in Mildred's room and the two fly off to the festival to turn Mildred in. Mildred gives herself away and barges up to Mr Hellibore where she gives him the box that contains Algernon. Mr Hellibore, who was one of Algernon's fellow magicians at the time of his transformation, turns him back to his human form.
As a last request from Mildred, Mr Hellibore magicks up a pot of tea along with some toast and crumpets with butter. The three head off to enjoy the supper.
Script error - Mr Hellibore previously appeared in The Worst Witch.
- The events that happen in this book are the cause for the events that happen in the next book The Worst Witch All at Sea.
- At the end of The Worst Witch All at Sea Algernon Rowan-Webb gives Mildred a necklace of two frogs shaking hands as a reminder of what happened in this book.
- This is the first and last time Sybil Hallow and her friend Clarice (whose surname is not given) appear in the book series. In the television series, however, they have much larger roles (Sybil was played by Charlotte Knowles and Clarice was played by Georgia Isla Graham). The TV series also gives Clarice's surname as Crow, making her name Clarice Crow.
- This book was also the first and only appearance of Griselda Blackwood. In the TV series, she is given a much larger role where she is friends with Fenella Feverfew (who was mentioned several times in The Worst Witch All at Sea but never actually appeared). In the episodes that televise A Bad Spell for the Worst Witch, Mildred kidnaps not Griselda but Ethel's friend Drusilla Paddock.