A couple of inspectors review an exhibit in the Black Museum, namely a handkerchief and a red leather bag that was the key evidence in a murder cae. The case dates back to 1941, and the little gasmask case that belonged to Philip Avery. Was Philip kidnapped? Searches in the countryside haven’t turned up anything yet. With dad dead in the war, and mom the single parent, there’s thinning hope that the boy will be found alive.
Mrs Avery turns out to be the sister of a Scotland Yard Inspector, and we learn that though her boy could be a handful, he wasn’t more than a strong willed child. In his own way concerned about being the man of the house. When Philip is found dead, mom feels the blame for his death. A fight in the last moments she spent with him, and for her own reasons, attributing the war and the army for the loss of her boy. A talkative woman provides a clue that ties the murder to an army convoy that passsed through recently.
The clues point to a convoy that is to ship out soon, and the race is on to catch up to it and see if the killer can be identified. The suspected soldier is a mild mannered man. Could he actually be the killer? The evidence points his way, but is he telling the truth, or about to be caught in a lie? A soldier goes AWOL, and when all the clues are put together, the killer is found, and brought to justice.