This wasn’t Matheson’s first work for Hammer. Back in 1957 he adapted his novel I Am Legend into a screenplay for Hammer that at the time was supposed to be directed by Fritz Lang. The British censors rejected the script and Hammer sat on the film until they sold the screenplay to an American associate Robert L. Lippert who had the screenplay reworked and eventual turned it into The Last Man on Earth, which hit screens a year before this movie. Matheson may have been dissatisfied enough that he removed his name from The Last Man on Earth but he was quite happy with Fanatic.
Patricia (Stefanie Powers) arrives in England to marry her future husband Alan (Maurice Kaufmann) but before she does she wants to visit the woman whose son she almost married but he died in a car crash. The woman, Mrs. Trefoile (Tallulah Bankhead) lives alone on a estate except for a couple of servants and Donald Sutherland. Mrs. Trefoile was an actress but now with her son dead has become very religious and considers Patricia to be her daughter-in-law even though she never married her son.
Patricia finds Mrs. Trefoile to be a bit hostile and makes the mistake of telling her that she was never going to marry her son and that he committed suicide. Then things get worse for Patricia and Mrs. Trefoile has servants to help out with her plans.
Tallulah Bankhead didn’t make a lot of movies, she was more of a theatrical actress but she was famous enough that her appearance in this film added some credit to Hammer which was becoming known only for their horror output. This was also one of Bankhead’s last movies. The movie itself is pretty good as Stefanie Powers tries to find a way out of Bankhead’s grip which creates a lot of suspense.