Once again Captain Hugh "Bulldog" Drummond (John Howard) is about to wed his long-suffering fiancée Phyllis Clavering (Louise Campbell) at her families' Swiss villa. The wedding is postponed when the Swiss police officer who is guarding their wedding gifts is killed. The murderer had a hidden agenda, as one of the gifts was an artificial diamond developed by the scientist father of the couples' friend Gwen Longworth. Drummond and his usual band of investigators must come to the rescue by tracking down the killer and uncovering their evil plot.