Tolefree is visited by Cossor, a K.C. whom he met on an earlier case. Cossor desires that Tolefree should journey to Cornwall and join a house party at Blackabrook, the home of the retired K.C. William Trethewy, and to investigate the theft of a missing necklace. Cossor claims that he is merely anxious to avoid any unwonted unpleasantness that might arise from the revelation of the culprit. But Cossor has deeper fears which he refuses to divulge until Tolefree is on the scene. Not long after Tolefree's arrival, the necklace is returned as mysteriously as it disappeared. But a particularly nasty attempt on Trethewy's life leads Tolefree to realise that his task has now become the more serious one of preventing a murder - before it is too late...