Holmes and Watson are visited by a lamplighter who claims his work is being sabotaged.
The lamplighter, Percy Wentworth, explains that some of his tools have been damaged or stolen. In addition, some of the lamps on his round are being extinguished during the night by an unknown hand, which could not only affect the safety of the general public but cause him to lose his job.
When Holmes asks if Wentworth knows who could be behind the troubles, the lamplighter gives them the name of a rival who has made no secret of his desire to have Wentworth's round for himself. He also tells them about a shadowy figure who has been following him on many occasions.
As Holmes and Watson begin their investigation, they discover that the person behind the mystery will stop at nothing to get what they want, even if that means resorting to violence.