Philadelphia police officers stopped a car driven by Harry Mimms (defendant) because the car had an expired license plate. Although this was a routine traffic stop and the officers had no reason to suspect criminal activity, they ordered Mimms to exit the vehicle. After Mimms exited the car, the police noticed a bulge in Mimms's jacket. The police frisked Mimms and found a revolver under his jacket. The State of Pennsylvania (plaintiff) prosecuted Mimms for carrying a concealed gun without a permit. The trial judge denied Mimms's motion to exclude the gun from evidence. The gun was introduced into evidence, and Mimms was convicted. Mimms appealed to the Supreme Court of Pennsylvania. The state high court reversed the conviction on the grounds that, because the police ordered Mimms to exit the car without any reason to suspect his involvement in criminal activity, the seizure of Mimms's gun was unconstitutional under the Fourth Amendment to the United States Constitution. The United States Supreme Court granted certiorari.