The young witch in training, Mildred Hubble, has a less than desirable start of her first year at Miss Cackle's Academy as she makes quick enemies with the prissy Ethel Hallow & the tyrannical Miss Hardbroom for arriving late on her first day! To make matters worse, Mildred quickly finds herself in deep trouble & is on the verge of being expelled!

The girls are told the legend of Sir Walter who died in a rainstorm centuries ago outside the castle. Whenever there is a storm Sir Walter is said to walk the corridors, looking for revenge! Now Mildred & the girls are out to see if the legend is true! Meanwhile the girls decide the quality of the food they're being served is poor & demand a change!

It's time for the presentation of the kittens which will sit decoratively on the end of each girl's broomstick when they fly! When Mildred's Tabby can't even cling on to her broomstick, Ethel lays into Mildred about how hopeless she is, Mildred has been pushed too far, & turns Ethel into a piglet! She escapes, & the race is on to catch it & turn her back, before Miss Hardbroom finds out!

When Mildred's attempts to revise her potion test notes fail, Mildred & Maud produce an invisibility potion by mistake, getting the duo into trouble! Mildred is determined lie low in an effort to stay out of trouble, but the Hallowe'en festival looms, & Mildred's class has been chosen to put on a flying display! A very cross Ethel decides to sabotage Mildred's broom!

Stopping to rest in the valley below Mildred comes upon a group of witches & hears them planning to take over Cackle's Academy! Mildred, thinking on her feet, turns all the evil witches into snails. Back at school, while Miss Hardbroom does not believe Mildred, Miss Cackle recognizes the description of the Chief Conspirator as her wicked & jealous twin sister, AGATHA.

Thinking that some extra responsibility might do Mildred some good, Miss Cackle asks her to look after the new first year, Enid Nightshade. Soon Maud feels Mildred hasn’t got time for her any more & goes off in a huff. To make matters worse, Enid sneaked her pet monkey into the school & he's on the loose! Now Miss Hardbroom thinks the roving monkey is Ethel & that Mildred is obviously responsible!