The year is 1645 - the middle of the English Civil War. Matthew Hopkins (Vincent Price), an opportunist and witchhunter, takes advantage of the breakdown in social order to impose a reign of terror on East Anglia. Hopkins and his assistant, John Stearne (Robert Russell), visit village after village, brutally torturing confessions out of suspected witches. They charge the local magistrates for the work they carry out.Richard Marshall (Ian Ogilvy) is a young Roundhead. After surviving a brief skirmish and killing his first enemy soldier (and thus saving the life of his Captain), he rides home to Brandeston, Suffolk, to visit his lover Sara (Hilary Dwyer). Sara is the niece of the village priest, John Lowes (Rupert Davies). Lowes gives his permission to Marshall to marry Sara, telling him there is trouble coming to the village and he wants Sara far away before it arrives. Marshall asks Sara why the old man is frightened. She tells him they have been threatened and become outcasts in their own village. Marshall vows to Sara, "rest easy and no-one shall harm you. I put my oath to that." At the end of his army leave, Marshall rides back to join his regiment, and chances upon Hopkins and Stearne on the path. Marshall gives the two men directions to Brandeston, then rides on.In Brandeston, Hopkins and Stearne immediately begin rounding up suspects. Lowes is thrown into a cell and tortured. He has needles stuck into his back (in an attempt to locate the so-called "Devil's Mark"), and is about to be killed, when Sara stops Hopkins by offering him sexual favors in exchange for her uncle's safety. However, soon Hopkins is called away to another village. Stearne takes advantage of Hopkins' absence by raping Sara. When Hopkins returns and finds out what Stearne has done, Hopkins will have nothing further to do with the young woman. He instructs Stearne to begin torturing Lowes again. Shortly before departing the village, Hopkins and Stearne execute Lowes and two women.Marshall returns to Brandeston and is horrified by what has happened to Sara. He vows to kill both Hopkins and Stearne. After "marrying" Sara in a ceremony of his own devising and instructing her to flee to Lavenham, he rides off by himself. In the meantime, Hopkins and Stearne have become separated after a Roundhead patrol attempts to commandeer their horses. Marshall locates Stearne, but after a brutal fight, Stearne is able to escape. He reunites with Hopkins and informs him of Marshall's desire for revenge.Hopkins and Stearne enter the village of Lavenham. Marshall, on a patrol to locate the King, learns they are there and quickly rides to the village with a group of his soldier friends. Hopkins, however, having earlier learned that Sara was in Lavenham, has set a trap to capture Marshall. Hopkins and Stearne frame Marshall and Sara as witches and take them to the castle to be interrogated. Marshall watches as needles are repeatedly jabbed into Sara's back, but he refuses to confess to witchcraft, instead vowing again to kill Hopkins. He breaks free from his bonds at the same time that his army compatriots approach their place of confinement. Marshall grabs an axe and repeatedly strikes Hopkins. The soldiers enter the room and are horrified to see what their friend has done. One of them puts the mutilated but still living Hopkins out of his misery by shooting him dead. Marshalls mind snaps and he shouts, "You took him from me! You took him from me!" Sara, also apparently on the brink of insanity, screams uncontrollably over and over again.