Five years after a mine disaster, Mrs. Peel and Steed are called upon to investigate the spooky goings-on at the local inn. The ghost of the 15th Duke of Benedict, who was reputedly lost in the mine disaster, has been seen wandering the area and Emma joins SMOG and FOG in a ghost hunt. The Duke's ghost supposedly takes Emma with him to his final resting place in the collapsed mine, but Steed descends into the shaft to rescue her. He finds a secret underground city that is being used to stage an invasion of England; the mine disaster was just a cover-up for the plan. Steed rescues Emma and country by cutting off access to the topside elevator.
—Marta Dawes