Charles Spalding investigates strange noises heard from the house of his near neighbour, Dr Franklyn. As he does so, a reptilian creature attacks and kills him. His body is dumped at the local cemetery by Franklyn's Malay servant and is buried the next day.

Charles' brother, Harry, arrives at the village after inheriting his brother's cottage. The landlord of the local pub, Tom Bailey, tells him that the locals don't like strangers and, due to his brother's misfortune, advises him to leave.

Harry and his wife travel to the cottage and upon entering see that it has been ransacked. In a fit of rage, Harry returns to the pub and demands to know who broke into his house, but to no avail.

Meanwhile, Dr Franklyn approaches Valerie, who is busy tidying the cottage, and enquires as to whether she has seen his daughter, Anna. She says no, and he retires to his house.

Harry returns from the pub and is attacked by a man on the way. He overpowers the man and discovers that the local villagers call him 'Mad Peter'. Harry invites him back to his cottage to gain some information, but Peter becomes agitated at the distant sound of pipe playing coming from the moors and flees.

Harry and Valerie are awakened in the middle of the night by the noise of Peter outside their house. Harry opens the door to find Peter frothing at the mouth and with his face blackened. Harry rushes to Dr Franklyn's house, and Franklyn reluctantly returns to Harry's cottage to discover Peter already dead. He discounts the strange symptoms as an epileptic fit, and confirms he will make the necessary funeral arrangements.

After the funeral, Valerie discovers Anna waiting for her at the cottage with a gift of flowers. Anna invites her and Harry to dinner that evening, but is then escorted away by Dr Franklyn, who is visibly angry about the dinner invitation.

Over dinner, Franklyn tries to persuade Harry to leave the cottage, but his advice is ignored. Franklyn commands Anna to play the sitar for their guests, but the tune angers Franklyn, who smashes the instrument and sends Anna away. Harry and Valerie leave.

In the morning, Harry arranges to meet Tom at the pub after closing time, and once there discovers Tom has dug up Peter's body. Tom reveals curious bite marks on Peter's neck, and persuades Harry to dig up his brother's body. They find the same marks on his brother's neck, coupled with the same strangely blackened face. Both recognise the symptoms as signs of a poisonous cobra bite.

The next day, the Spaldings receive a note from Anna, pleading for their help. Harry goes to the Franklyn house and is attacked by a woman who has the appearance of a snake. Harry manages to return to his cottage and is saved from certain death when Valerie cuts off the wound. Back at Franklyn's house, the Malay servant slaps Franklyn around the face and tells him to follow and kill Harry.

Valerie summons Tom to the house, and he deflects Franklyn's attentions by telling him that Harry has died. Valerie sets off to Franklyn's house while Franklyn himself is preparing to kill Anna in the basement where she hibernates. Before he can strike her with a sword, his servant tries to fight him, but Franklyn pushes him into a sulphur stream that runs underneath the house. As he falls, the oil lamp he was carrying overturns, and the house is set alight.

Franklyn discovers Valerie and locks her in his study. He reveals how the snake people of Borneo cursed Anna after they became offended by his investigations into their religion, and how she is now condemned to periodically become a snake.

As Franklyn leaves Valerie to die, Anna attacks and kills him, and Tom breaks the glass in the study window to distract her from biting Valerie. Harry rushes in and rescues Valerie, but Anna, unable to cope with the cold air coming in from the broken window, collapses and is left to perish in the fire.