I Married a Witch begins in colonial Salem, Massachusetts. Puritans burn two witches, Jennifer and her father Daniel, at the stake. They then bury their ashes under an oak tree to trap their evil spirits for as long as the tree shall live. Before they are burned, Jennifer curses Jonathan Wooley and all his male descendants to a life where they always to marry the wrong woman.
Over the next 270 years, generation after generation of Wooley men fall for the most despicable women imaginable until lightening from a storm in 1942 strikes the old oak tree. The explosion splits the tree, and frees Jennifer and Daniel in the form of wisps of witch’s smoke.
They float to a nearby mansion where they find Wallace Wooley running for governor. The next day he is to marry the uptight Estelle Masterson. Her father, J.B. Masterson, is a political backer of Wooley’s, and J.B. never backs a loser.
Jennifer tells her father about the curse she put on the Wooley family, and Daniel replies that a better curse would be to have them fall in love with women they can never have. This thought delights Jennifer, and she convinces Daniel to give her a human body so she can make Wooley fall in love with her.
Daniel needs a massive fire to cast a spell to give her one, so he burns down the Pilgrim Hotel. Jennifer uses the opportunity to have Wallace rescue her from the flames. She attempts to seduce him using her womanly charms and without witchcraft, but it doesn’t work. Wallace refuses to cancel his marriage.
She creates love potion, and tries to get him to drink it, but as she does, a painting falls from the wall, and hits her on the head. Wallace gives her the potion to drink, thinking it’s water, and falls in love with him instead in a bewitching twist of fate.
Daniel burns down another building to create himself a body. He helps Jennifer crash the wedding, but his motives are different for doing so than Jennifer’s. Daniel hates the Wooley family for imprisoning him, but he gets drunk during the wedding and falls from the balcony. The police arrest him, and throw him in the drunk tank. Unfortunately, the alcohol causes him to forget all his spells to free himself.
After Daniel’s fall, Estelle and her father find Wallace kissing Jennifer, and she calls off the wedding. J.B. threatens to use his wealth and position to ruin Wallace’s chances of winning the gubernatorial election the next day.
Wallace admits he loves Jennifer too, and they escape a thorough tar and feathering to elope in the countryside. After the quickie marriage, Jennifer admits she’s a witch, but Wallace doesn’t believe her. To prove it, she conjures a brainwashing cloud to trick the populace into voting for Wallace. He wins in a landslide, and because his opponent receives no votes, it convinces him that Jennifer is telling him the truth.
Now free and angry, Daniel strips Jennifer of her magic, and sets out to force her back into the oak tree until she comes to her senses. Jennifer panics, and interrupts Wallace’s victory speech. They take off in a taxi ride to anywhere, but Daniel is two steps ahead. He’s the cabbie, and he drives them into the sky, and back to the oak tree.
At midnight, Jennifer leaves her body to join her father at the tree. Wallace picks up her lifeless body, and heads back to the mansion. Jennifer convinces her father to follow him so they can enjoy his misery.
Wallace lays Jennifer’s body on a couch while Jennifer and Daniel watch on as wisps of smoke. As Daniel delights at the site of Wallace’s sadness, and hops into a bottle of whiskey. With dad distracted, Jennifer returns to her body, and tells Wallace that love is stronger than witchcraft. She sneaks back to her father, and corks the bottle; trapping him inside, and keeping him drunk…rendering him powerless.
Fast forward about seven years, and Wallace and Jennifer have three children. While the boys seem mortal, their little girl rides around the room on a broom. Their housekeeper complains that she can not control this one. The films ends with a happy but uncertain future.