Christopher's distinguished cousin, Sir Gordon Race, is defending Lady Mendel, on trial for the murder of her husband. On the morning of the last day of the trial, a veiled woman visits Christopher and gives him a package, saying it contains a gift for Sir Gordon which will bring him luck. She wants Christopher to take it for her. He consents and arrives at Sir Gordon's in time to meet his cousin's fiancée, Miss Collingwood, and her mother. When Christopher gets home again he finds a note warning him that the package contained a watch filled with a powerful explosive. He rushes back to Sir Gordon's, but finds him gone. However, he catches him before he enters the court. He learns that Sir Gordon has given the watch to his fiancée, who has gone hunting. Christopher promises to follow in Scarlet Runner to catch and warn them. He persuades Sir Gordon to remain and defend the woman on trial. The bomb mystery clearly verifies one suspicion in Sir Gordon's mind and he goes at the defense from another angle. He succeeds in breaking down the testimony of Miss Mendel, who has accused Lady Mendel, and in implicating her and her secretary, who, terrified by the fierce questioning, breaks down and confesses. Christopher overhauls the Collingwoods. He arrives at the house where they are stopping in time to see a burglar attempting to escape with the watch. The explosion comes and Christopher is injured. He recovers in time to share in the happy reunion when Sir Gordon arrives from his successful trial.