The Mists of Time is the eleventh episode of The Worst Witch (2017 TV Series). It was first broadcast on CBBC and CBBC HD on 15 March 2017. It is preceded by Spelling Bee, and followed by Out of Bounds.
The Academy is shrouded in mist. Miss Cackle says that it is the 'Mists of Time', and orders the school to be locked down and all the exits sealed. However, Ethel's cat, Nightstar, is still outside. Mildred says she knows a way out, and they sneak out through the basement.
In class, Miss Drill realises Ethel and Mildred aren't there, and tells Miss Cackle and Miss Hardbroom. Miss Hardbroom says that as long as they come back into the castle the same way they left, it won't be a problem. Mildred and Ethel, unable to find Nightstar, decide to come back in, but the cooks are in the basement, so they use another way.
Inside, they encounter a girl who looks like Maud, but who says her name is Mona, and she doesn't recognise them. They also encounter twin girls who are going through Miss Cackle's office. They don't know who Miss Hardbroom is, and insist that the headmistress is Mrs Alma Cackle, not Miss Ada. Alma then comes into her office, catching the girls, who she addresses as Ada, Agatha and Spellbody. Mildred and Ethel realise that they've gone back in time, as the girls are Miss Cackle, her sister Agatha, and Maud's Grandma Mona.
Meanwhile, Alma tells Ada, Agatha and Mona that they'll be expelled if she catches them going through her office again. In an empty classroom, Mildred finds a book which explains that the Mists of Time is a magical mist which links two points in time, but only until it clears- after that, you're stuck in time. Mildred writes a message on the underside of a desk lid, hoping people in the present will find it and help them.
A teacher that looks like Miss Drill (perhaps her mother?) is taking a class photo of Ada and Agatha's class. Mildred and Ethel sneak into line and ask Ada, Agatha and Mona for help. They agree, as long as Mildred and Ethel help them first. They want to get Alma's spellbook from her office, so they can use her power to run the school how they want, on the basis that they're going to be Headmistresses anyway, they're just speeding things up.
In the present, Maud finds Mildred's message in her desk, and Miss Drill goes to tell Miss Cackle and Miss Hardbroom. Meanwhile, in the past, Mildred and Ethel go into Alma's office and find the spellbook, and Mildred also takes a scroll she finds. Ada and Agatha run off with the spellbook, while Mildred tells Mona that she's friends with her granddaughter. Alma then comes in, and Mona is expelled for being in her office. Meanwhile, Agatha has decided to take their mother's power and run the Academy herself, despite Ada's objections.
In the present, Miss Hardbroom says desk graffiti isn't evidence that Mildred and Ethel are in the past, but Miss Cackle points out that they are in her old class photo. Meanwhile, Mona's expulsion in the past effects Maud, and she becomes naughty and bad tempered, refusing to do her work. Miss Cackle goes to the past and finds Mildred and Ethel. she tells them to go outside and then back in the basement in order to get back to the present.
As Ethel returns to the present, Mildred shows Miss Cackle the scroll she took from Alma's office. It's the twins birth scroll, and it shows that Agatha is the eldest (by 13 minutes), not Ada as they believed, meaning that Agatha was supposed to inherit the Academy. Miss Cackle then goes back to the past to deal with herself, Agatha and Mona. Mildred follows her back and leaves another message in Maud's desk. Miss Cackle tells her younger self and Agatha who she is, and attempts to talk Agatha out of her plan, to prevent her from getting expelled and sent to Wormwood's Academy (where wicked witches are sent) while Ada becomes head girl, and their attempt to run the school together will be a disaster. However, the young Ada doesn't want to be head girl, and grab Agatha's bag of ingredients, intending to carry out the plan herself.
In the present, Maud finds Mildred's second message, which says to tell Miss Drill what Miss Cackle is doing, but being mean, she ignores it. In the past, Mildred, Mona, Agatha and Miss Cackle find Alma and tell her what's happening. They find young Ada, and Mildred talks her out of changing things, telling her about the kind and supportive person she becomes as headmistress.
Agatha says that she knew Ada wouldn't go through with it, and casts the spell on the potion herself, causing it to explode. In the present, Miss Hardbroom realises that Mildred and Miss Cackle are still missing, and Maud and Drusilla team up to tease Ethel, calling her 'Ethel Shallow'. In the past, Alma tells Ada that she lied about which twin was older as she wanted Ada to have the school, because Agatha would never run it according to the proper values. Mildred also tells Alma not expel Mona, and Alma tells Agatha she will be going to Wormwood's.
In the present, Maud and Drusilla return to normal, and Ethel realises Mildred has fixed things. Miss Cackle and Mildred race to get back to the present before the mist goes, but on the way Mildred finds Nightstar, who has found Ethel's Grandmother. Miss Cackle gets back as the mist clears, and everyone thinks Mildred is stuck in the past, but then she comes in with Nightstar.