Episode 18, "The Invisible Knife" stands out as one of the best, as Colonel March is retained to protect the notorious Basil Pennacott (Hubert Gregg), who has profited from the deaths of 5 different partners in Singapore, Bombay, Suez, Casablanca, and Tangier. 4 were made to appear as natural deaths, but the fifth, Edmund Hays, was very mysterious indeed- murdered by an invisible assailant with an invisible knife. Pennacott insists that the superstitious Hays was killed while attempting to summon The Devil during a black rite, and now the man's brother (Leslie Weston) has vowed to avenge him by killing Pennacott. A dead parrot (shades of Monty Python!), its beak dipped in poison, was apparently sent from a pet shop run by Hays and his daughter Alice (Henryetta Edwards); March seeks to convince them both that he can deal justice to the ruthless Pennacott, not wishing to see bitterness destroy their lives. The solution is a genuine surprise, even if the supernatural really isn't involved. Another surprising bit of irony is that Hammer veteran Terence Fisher directs his only entry, among the nearly three dozen TV episodes he did from 1952 to 1959.