London helps a struggling magician named Elmer, who is the unsuspecting dupe of diamond thieves.
The thieves arrange for Elmer to use his disappearing act on a diamond at an auction, and they seize the diamond in the middle of Elmer's trick and replace it with a cheap copy.
Elmer is arrested, and the thieves think that they have successfully stolen the gem, with Elmer as their "patsy", but they had not considered the London factor!
London follows them to their apartment, snatches the jewel from them, brings it to Elmer's domicile, and drops it in an aquarium.
Unable to find the diamond, the thieves post bail for Elmer's release from police custody and promptly force Elmer at gunpoint to command London to reveal the gem's hiding place.
London cooperates, and the criminals procure the diamond from the aquarium, but they are prevented from escaping after London attaches a metal cable to the rear of their car, causing the car to abruptly stop.
Police, in the vicinity of Elmer's home to maintain observation of Elmer, surround the thieves and retrieve the jewel, and Elmer is "cleared" of larceny charges.