Members of the British Venusian Society, an organisation set up to observe the planet Venus, are literally and fatally turning white while on watch at night! A phone call from Mother sends Steed and Mrs. Peel off to investigate this phenomenon. Mrs. Peel makes contact with Bert Smith, an upper-class chimney sweep and member of the society, but before he can give her much information, he is killed by a blinding white flash. Meanwhile, Steed joins the BVS (whose secretary is the aptly named Venus Brown), and is sent for an eye test with the society's optician, Dr. Primble. Mrs. Peel tracks down another member, Lord Mansford, but he too is killed before she can interview him.Steed, now a member of the BVS, goes to see Brigadier Whitehead (another member), shortly after Venus Brown has paid the old soldier a visit. However, once again he gets there too late - only managing to hear the strange whooshing sound as it precedes a blinding flash. Fortunately, the Brigadier was in the middle of taping his war memoirs and since the recorder was left running it has captured the sound of the lethal ray, providing Steed with another clue. Mrs. Peel, pretending to be a journalist, visits the BVS and plays the tape to Venus and Crawford (the society's radio astronomer) hoping that they'll incriminate themselves in the deaths of the other members. However, it becomes apparent that they are not involved, and Venus herself only narrowly escapes being zapped by the "Venusian" ray whilst Mrs. Peel is there.