Father Unwin discovers that a gang of bogus vicars are responsible for a series of sabotage attacks on military installations and vehicles. After one ofthe gang is hospitalised after their latest venture, Unwin masquerades as his replacement, a demolition expert, to infiltrate the gang, but he is soon exposed as an imposter.

Lord Edward and Lady Martha Hazlewell have inherited a counterfeit plate from their father, but the location of the second plate is known only to their father's accomplice, Mullins. Unwin and Matthew trail Mullins to Hazlewell Manor and join a frantic chase to retrieve the second plate from an old barn at Greenacre Farm.