Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a mediocre old English horror movie. It's mostly talking with little action. Nothing about it is very frightening either, except for the ignorance of the screenwriter and the rural villagers regarding snakes. Even though there's a woman who's supposed to be a shape-shifting snake, we hardly see her, and worse than that we never see any snakes attack. We just see brief shots of snakes and either a hand with two fang holes in it, or a dead body. It looks like all the snake woman has to do is stare at someone then they drop dead. Maybe that was terrifying in 1961, but not now.
In a small village on the bleak moors of Northumberland in 1890, a herpetologist, Dr. Adderson (get it - adder's son), milks venomous snakes to use their venom in a medicine he makes and injects into his pregnant wife Martha to cure her "sick mind." She dies in childbirth, and her daughter Atheris is born unblinking and cold-blooded. (Atheris is a genus of tropical African vipers.) The old and superstitious midwife claims that the baby is cursed, and alerts the village that Martha was murderded by the baby when she looked into her eyes. The doctor who delivered her takes Atheris to a shepherd's house, telling him that Dr. Adderson will pick her up the next day. Men of the village come to the house, smash the snake cages, and burn the house down. (Because that's how men solve all problems.) Dr. Adderson tries to rescue some of the snakes in the burning house, but he is bitten by a venomous snake and dies. The doctor who delivered the baby leaves for Africa without knowing Dr. Adderson is dead, leaving the shepherd to raise the baby snake girl until she grows up and disappears.
Twenty years after Atheris' birth, the village has been suffering every year from deaths by snakebite, so a Scotland Yard detective is sent there to find out why. He meets a Colonel who spent years in India. When a village man dies from snakebite, the Colonel looks at his wound and tells the detective that the bite is from a King Cobra. He saw many bites like it in India. He gives the detective a snake charmer's flute that gives anywone who plays it the power to attract snakes.
The detective wanders out on the moors at night and plays the flute then meets a beautiful young woman. She tells him she was attractive to the music. He plays the flute again and a snake appears, so he raises his gun to shoot it, but she stops him, telling him it has a right to live. This looks like a romantic scene to the detective so he tells her he wants to see her again tells him he will see her again if he plays the flute and then she leaves. Later, the detective finds a huge woman-sized pile of shed snake skin, which he picks up. (This part makes no sense. Women don't shed that way. They'd like to, with all their expensive exfoliants and dermabrasions, but they don't. If she changes into a snake, she would shed as a snake.) After some voodoo doll nonsense with the old midwife who convinces him that Atheris is the snake woman, the detective goes back out to the moors again and sees a cobra. This time he shoots the cobra, and we see Atheris' dead body fading in where I assume the cobra was. (We don't even see the dead cobra.) But this finally gives us some evidence that she does change into a snake and back. That was never too apparent. Before that, we saw her appear and disappear quickly, probably from and back into a snake. But we also saw other snakes while she was in human form. I had to assume that there were snakes that were offspring of the snakes that escaped 20 years earlier, even though there's no way tropical snakes could survive in the wild in Northumberland. And the question remains, who was biting the villagers, her or the other snakes? We'll never know for sure, even though the advertisement poster claims she turns into a deadly reptile at will.