Agatha Cackle is the main antagonist of Season 1 of the 2017 BBC series, The Worst Witch
She's played by Clare Higgins in the 2017 reboot series, and Claire Gover as Young Agatha also in the 2017 series.
Agatha turns up, and Miss Cackle gives her permission to speak to Miss Hardbroom for an internet article. Mildred then witnesses Agatha putting a spell on the celebratory meal. When Mildred tries to warn the staff, she is magicked outside the school grounds. With the help of a magic frog she levitates herself back into the school, where she manages to prevent Miss Cackle eating the soup, announcing Agatha put a spell on it.
Agatha invokes a section of the Witches Code and she and Miss Cackle have a magical duel. Agatha wins and turns Miss Cackle into a snail. After Miss Hardbroom pointedly drops hints (in Midred's direction) about how students or staff are bound by the witch's code and can do nothing about Agatha now being in charge, Mildred prompts the cat to knock the soup with Agatha's spell - which was a total obedience spell - over Agatha and then orders her to turn Miss Cackle back to normal. Agatha's powers are then confiscated.
The Great Wizard's VisitEdit
Agatha masquerades as Miss Cackle again, while. The real Miss Cackle is in an enchanted sleep on the bed. However, when Mildred's potion display gets out of control and The Great Wizard is splashed with potion, he announces that he will be removing Miss Cackle as headmistress, and the school will go to the next Cackle in line - Agatha. However Miss Hardbroom is able to talk him out of it with a speech in support of Miss Cackle.
The Mists of TimeEdit
Mildred and Ethel accidentally go back in time, and meet Ada and Agatha as schoolgirls, and their mother Alma as headmistress. Agatha and Ada try to take their mother's power and run the Academy themselves, however Mildred and the Miss Cackle from the present talk them out of it. It turns out Agatha is the eldest (by 13 minutes), not Ada as they believed, meaning that Agatha was supposed to inherit the Academy. Their mother told 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.
Agatha ends up getting expelled and sent to Wormwood's Academy (where wicked witches are sent) while Ada becomes head girl. They apparently attempted to run the school together, but it was a disaster.
Out of BoundsEdit
Agatha escapes her banishment at Darkwood Cottage, and pretends to be Miss Cackle again. She claims Agatha has taken her powers and tries to get Mildred to give her hers, but Mildred realises that it's actually Agatha. Agatha and Miss Gullet then turn up at the Academy, and claim that Agatha is Miss Cackle. Esmerelda invites them in, and is tricked into giving Agatha her power.
The Worst HeadmistressEdit
Agatha and Miss Gullet terrorise the school; they turn Miss Drill back from a snail, but turn Miss Bat and Algernon into frogs and Esmerelda into a trophy, while Ethel is made head girl. Agatha also casts enchantments so that no-one can use magic without her knowing. Agatha also turns The Great Wizard into a balloon, and traps Miss Cackle and Miss Hardbroom in a painting.
The students stand up to Agatha, saying that if Agatha is going to be Headmistress, they will all quit so she doesn't have anything to be Headmistress of, and everyone agrees. Agatha decides that if she can't have the school, then no one will, and casts a spell to destroy the castle. When Agatha tries to destroy the school, All the students chant together to stop her spell, and this also changes Esmerelda Hallow, the teachers, and the grand wizard back into themselves, and Agatha and Miss Gullet end up trapped in a painting by Miss Cackle.