A mother was left shaken after a man jumped into her car and drove it at speed towards the centre of Aylesbury while she was sat in the back with her baby.
The shocking incident took place in town on Tuesday October 2.
At around 8.40pm the victim, a 22-year-old woman, was sat in the back seat of her car, a white Seat Leon, with her young baby, in the Nisa Local car park on Buckingham Road.
While she was waiting for her husband to return, a man jumped into the driver's seat and threatened her with a knife.
He then drove out of the grocery store's car park, towards the town centre, at high speed and in a manner police described as 'erratic.'
The woman hit the man around his head and he stopped the car in the middle of the road by the Whaddon Chase junction before running off down Whaddon Chase.
The victim's car is a white Seat Leon on a '15' registration plate.
Detective Constable Carolyn Bailey said: "The victim wasn't injured, but was left shaken after this incident.
"I am keen to speak to anyone that might have seen the incident, or may have seen the offender running away from the car.
"If you were in the area around the time of the incident and have a dashcam I would ask you to please review the footage on it to see if anything has been captured.
"If you have to wait for someone in your car we would advise you to lock the vehicle and remove the keys from the ignition."