The screenplay, by Doctor Who writers Mervyn Haisman and Henry Lincoln,[2] was based (uncredited) on the short story "The Dreams in the Witch House" by H. P. Lovecraft.

[3][4] Antiques dealer Robert Manning searches for his brother, who was last known to have visited the remote house of Craxted Lodge at Greymarsh, their family's ancestral town.

In an effort to kill both Robert and Eve, the modern and righteous descendant of Lavinia, Morley sets fire to Craxted Lodge but is trapped on the roof.

The house used for Craxted Lodge is Grim's Dyke, the allegedly haunted former home of William S. Gilbert, located in Old Redding, Harrow Weald, Middlesex, London.

[citation needed] Roger Greenspun of The New York Times wrote, "Karloff himself, cadaverous and almost wholly crippled, acts with a quiet lucidity of such great beauty that it is a refreshment merely to hear him speak old claptrap.

Nothing else in The Crimson Cult comes close to him – though there is Barbara Steele in greenface playing Lavinia, a glamorous 300-year-old and a monumental cast that lists no fewer than seven-party girls, plus several sacrificial virgins.

"[10] Kevin Thomas of the Los Angeles Times called the film "a delight for horror fans, with Karloff in top form despite the infirmities of age.

"[11] The Monthly Film Bulletin wrote that apart from a wild party and some exposure of a woman's bosom in a bed, "this is one of the lamest and tamest horrors in a long time, with the script hobbling along like an underprivileged Agatha Christie thriller through acres of would-be sinister dialogue as the handsome hero investigates endlessly and Karloff and Christopher Lee dispense meaningful sneers.