From IMDb: A pretty Chinese woman, seeking help from San Francisco detective James Lee Wong, is killed by a poisoned dart in his front hall, having time only to scrawl "Captain J" on a sheet of paper.

She proves to be Princess Lin Hwa, on a secret military mission for Chinese forces fighting the Japanese invasion.

Mr.

Wong finds two captains with the intial J in the case, neither being quite what he seems; there's fog on the waterfront and someone still has that poison-dart gun...

Date : 1939
Cast : Boris Karloff as Mr.

James Lee Wong
Marjorie Reynolds as Roberta 'Bobbie' Logan (reporter)
Grant Withers as Police Capt.

Bill Street
Huntley Gordon as Mr.

Davidson (bank president)
George Lynn (actor) as Capt.

Guy Jackson (Aviation Corp.

president) (as Peter George Lynn)
William Royle as Capt.

Jaime (captain, Maid of the Orient)
James Flavin as Police Sgt.

Jerry
Lotus Long as Princess Lin Hwa (murder victim)
Lee Tung Foo as Willie (Wong's servant) (as Lee Tong Foo)
Bessie Loo as Lilly May (Princess Lin Hwa's maid)
Richard Loo as Tong chief
Ernie Stanton as Burton (Davidson's butler)
I.

Stanford Jolley as Hotel clerk