About the author (Goodreads): Richard Horatio Edgar Wallace (1875-1932) was a prolific British crime writer, journalist and playwright, who wrote 175 novels, 24 plays, and countless articles in newspapers and journals.
Dick Allenby, inventor of a silent air-powered gun
Gerald "Jerry" Dornford, of Half Moon Street, man about town, gambler
Mike Hennessey, theatre manager
Mary Lane, actress, fiancee of Dick Allenby
Leo Moran, of 17 Naylor Terrace, banker and speculator
Horace Tom Tickler, burglar
Washington Wirth, a party-giver with an eye for the young ladies
Hervey Lyne, rich, moneylender, disabled, guardian of Mary Lane, uncle of Dick Allenby
Binny, Hervey's assistant
Chief Inspector Surefoot Smith, C.I.D.
Synopsis: Horace Tom Tickler, a small time burglar, is hanging around rich Hervey Lyne's place, plotting to get in. Chief Inspector Surefoot Smith encounters him and moves him along. Later that night, Smith is looking for a cab for put actress Mary Lane, in - and the cab he chooses has Ticker dead inside; and the cab is found to have been stolen. An anonymous note suggests he go talk to banker Leo Moran to find out about the killer.
Moran has been dealing in some shady financial transactions, and flees to parts unknown. While Surefoot investigates, he comes across a curious large silver key. Hervey Lyne, while in the park in his Bath Chair (type of wheelchair), is shot.
Mary Lane takes on the role of a private investigator and sets out to find where the silver key fits, and runs into danger.
Edgar Wallace has thrown in plenty of traditional mystery plot elements: secret rooms, secret identities, secret keys, disguises, stolen cars, and secret hideaways. Surefoot plods along, making progress; but Mary Lane takes the initiative to go find the lock to which the silver key fits. An enjoyable Wallace, with the culprit being revealed well before the end; then the task is finding him!
The vacant room which Surefoot locates - having just the silver key and a wardrobe of clothes - is a similar plot element to Ellery Queen's Halfway House. which would follow in 1940.