The Case Of The Thistle Killer is a Sherlock Holmes television movie from 1955.
It stars Ronald Howard as Holmes, H. Marion Crawford as Dr. Watson and Archie Duncan as Inspector Lestrade.
Each night in London, a woman has been murdered and three thistles left at the scene of the death.
It baffles the police but Holmes works it out, even to the extent of where the next crime will be committed.
It turns out that the killer wears a police uniform, which is why the women trusted him.
He was a rejected police officer and the three (feet to a yard) thistles (Scotland) was his way of taunting the police.
Movie pages here.