William Shatner's A Twist in the Tale - Plot Synopsis: The Magician

However, the Watsons’ lives change forever when they rescue the beautiful and mysterious Grizelda. Grizelda is a newcomer in what appears to be just a run-of- the-mill New England town - but since her arrival, the old aunt she says she came to visit has never been seen, and the townspeople believe she has been murdered. Sam and Emily find Grizelda fleeing for her life. She explains that she, like Sam, is a healer, and is being accused by the villagers not only of murder but of witchcraft.

Unused to sharing her father’s attention, Emily is glad to arrive at Bitter Creek, the little town they visit every summer, and to meet up again with the friend she made there the previous year, the farmboy Tom (14). But it is at Bitter Creek that Grizelda’s true nature will be revealed ...
Sam has none of Emily’s misgivings. In fact, Sam is a man in love - and to Emily’s distress, they no sooner arrive at the market and set up their stall than her father and Grizelda are married. From this point on - her objective achieved - Grizelda has no further interest in trying to win Emily over. Angered by Pepper’s protectiveness of his mistress, Grizelda makes the dog "vanish" during the magic show. Emily searches the tent after the show - and to her great shock finally finds a frog: a frog which barks.
So Emily does spy on Grizelda, and uncovers the ghastly truth. Far from murdering her old aunt as she was accused of doing, the luscious and youthful Grizelda is the crone herself. Her "headaches" indicate that the spell is wearing off - which is why whenever they strike she must rush back to the caravan to take a fresh dose of potion ...

Without the potion, Grizelda of course reverts back to her true shape. In a peculiarly ironic twist, Sam is accused of killing his lovely young wife and daughter - for where are they now ...?
Tom carries back news of Sam’s trial to Emily, who is hiding in his barn. As the children set off to the rescue, the barn door swings open to reveal, triumphant, Grizelda - she has followed Tom back from the village to Emily and the potion!
But the witch is no match for the ingenuity of the two children and Pepper. It seems only just that it is Grizelda’s own wand which transforms her into a harmless black cat - running for its life before the dog she so cruelly transformed.