The serial is about an FBI agent named Jerry Blake who battles gentleman thief Jim Belmont, who escapes custody with help of his gang and begins a wave of crimes, beginning with plotting to steal the crown jewels of the Princess Cornelia.

Crime lord James 'Jim' Belmont (George J. Lewis) escapes FBI custody and resumes his criminal empire, only to be thwarted at every turning point by British-accented Jerry Blake, the FBI's Operator 99 (Marten Lamont).

Belmont plots to steal the crown jewels of the Princess Cornelia, with the aid of his cohorts Matt Farrell, Rita Parker and his crafty secretary Morton.

The criminals succeed in stealing the jewels, then offer to ransom them back, using Jerry Blake (Operator 99) as the go-between.

Blake is able to trace Belmont to his hidden lair beneath a theatre and winds up battling him high up on a catwalk overlooking a precipitous drop.