The Limping Man plot
"What Strange Secret Walked Side By Side with... The Limping Man"
American Frank Prior arrives in London to visit his old flame Pauline French, whom he hasn't seen in six years. At the airport, a man standing next to Prior is killed by a sniper, who manages to get away undetected. Frank is being held by Inspector Braddock of Scotland Yard for questioning. After Frank is released, he goes to Pauline's apartment and learns that she had an intimate relationship with the murdered man, but does not want to discuss it further. Scotland Yard knows that too and they shadow Frank and Pauline.