Dr. Harvey, a rural physician, breaks the fourth wall to relate an unusual occurrence that happened in his village. The bulk of the story is told in flashback. Bill and Robin are boyhood friends who compete for the affections of Lena, a beautiful girl about their own age. Lena's family moves away, and in adulthood the two men become scientists. They collaborate on the Reproducer, a machine that can exactly duplicate physical objects. Lena returns to the village, and Bill and Robin's forgotten childhood feelings return. In time, they abandon their work on the Reproducer, and Robin leaves the village to learn his family's business. Bill is disappointed to discover that Lena loves Robin and intends to marry him. Hopeless that he can win Lena's affections, Bill convinces her to allow him to use the Reproducer to create a duplicate of her. The experiment succeeds, and Bill names the duplicate "Helen". Because Helen is an exact copy, when she is introduced to Robin she also falls in love with him. Bill believes that electro-shock therapy can be used to erase Helen's knowledge of Robin. Not wishing to compete with Lena for Robin's affections, Helen agrees to the therapy. Bill convinces Lena to help him with the procedure. The process proceeds as planned, but the apparatus overheats, explodes and causes a terrific fire. Robin and Dr. Harvey arrive in time to rescue a woman from the fire. Bill and the other woman perish in the flames. Harvey, having been briefed on the situation by Robin, discovers that the woman has amnesia. The two men wonder whom they have saved. Dr. Harvey recalls that Bill had had to start Helen's heart with a device that he attached to the back of her neck, leaving two scars. Robin is relieved to find that there are no marks on the neck of the woman they rescued: It is Lena.
Production details
Four Sided Triangle was an early effort by Hammer Films. The laboratory set includes "a welter of retorts, alembics, rheostats and plain, old neon tubing". This chaotic, improvised laboratory setting has been contrasted with the sophisticated labs portrayed by Universal Horror pictures of the 1930s. The picture relies on a minimum of trick photography and special effects, which may have been compromised by its limited budget.
Four Sided Triangle features some differences from the original novel by William F. Temple. In the novel, the duplicate falls into depression for being married with Bill while she's in love with Robin. She has a breakdown and has to go on holiday with Bill to recover. When they return, Bill starts working on a power generator which explodes, killing him. Lena tries to convince Robin to accept both her and Dot, but he refuses. A couple of weeks later, Lena and Dot have an accident while diving in a river. One of them dies and the other is seriously injured. Dr. Harvey and Robin are startled when they discover that the surviving woman can't recall anything after the duplication, and suppose she's repressing all the painful memories, so she must be Dot. Dr. Harvey finds in Bill's notes about the marks on Dot's neck and tells Robin, convincing him that the survivor is Lena.In an epilogue, he reveals that he also discovered a note in which Bill recalls that, during her holiday, Dot had undergone plastic surgery to erase the marks, but destroyed it so Robin and Dot can be happy.
