In CCTV footage from a 7-Eleven convenience store in the city of West Covina, a suspect points a gun at the clerk and demands he open the cash register.
As the employee finishes placing money into the plastic bag, Sergeant Hernandez of the West Covina Police Department walks in.
The clerk quickly alerts the officer of the situation and Sgt. Hernandez is seen chasing the suspect out of the door.
He arrested Ramon Gonzalez, who was booked for robbery, felon in possession of a firearm, and carrying a concealed weapon.
All the stolen cash was returned.
The incident happened on Wednesday 17 January.