The Cast*:

Sidney Toler: Charlie Chan
Arleen Whelan: Brenda Hartford
Richard Derr: Carl Detheridge
Douglas Dumbrille: [Paul] Manderley
Henry Daniell: Watson King (also known as Jim; alias Cesare Borgia)
Edmund MacDonald: Walter Hartford
Sen Yung: Jimmy Chan
Lenita Lane: Lucy Manderley

   (formally known as Lucia and Princess Lucrezia della Borgia)
Ethel Griffes: Madame Saturnia (also called Lizzie)
Milton Parsons: [Arthur] Fletcher
Steve Geray: Dr. Retling
Lucien Littlefield: [Professor] Gleason

George Chandler: Bus Driver (not credited)

Oliver Prickett: Hank (not credited)
Eric Wilton: Wilson (not credited)

Paul Kruger: Servant (not credited)

Edgar Norton (not credited)  

*Courtesy of www.charliechan.info