On robbery detail, Joe Friday investigates a rash of theft at successful retail shops. A donut shop owner is interviewed about his injuries, and the man in the mask who robbed him. The tiny gunman seems to be bulletproof, and stronger than he looks. The crime lab gets involved, and the other witnesses questioned has added little. The crime spree continues for months. The thief seems to have a knack for striking where the cops are not staked out.
More weeks go by with the thief robbing and kidnapping his victims under the noses of combined forces of cops. The ruthless, and mean old bandit seems to at least care enough to not kill his victims, and phones the cops to tell where the kidnapped shop owners can be found.
A lucky break finally puts Joe Friday and Frank Smith in the path of the robber. A gun battle sends the market bandit fleeing, injured, and on foot. The dragnet is thrown out as cops comb the area. Another short gun fight finds the man in the hands of Joe Friday.