Officers from Bedfordshire Police are appealing for information after the man and woman were targeted in the early hours of Friday.
At around 1am on Friday (24 June), the victims were in a car park between Althorp Road and The Ridings, Luton, when the vehicle was approached by two men.
One of the offenders ordered the driver to get out of the car, but when he refused the offender then took out a knife and punched him twice.
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The same man then stole a bag containing a number of items, including bank cards, before the pair made off.
The offenders are described as Asian, with one over six-feet-tall and the other slightly shorter. Both are of slim build.
Detective Constable Jordan Lancaster, investigating the crime, said: “This was an unprovoked robbery which targeted the innocent occupants of a vehicle.
And he urged anyone who knew anything to get in touch. “Although this took place late at night, I’m hopeful that anyone in the area at the time may have noticed the two offenders involved. I would urge anyone with information to get in touch right away,” he said.
Anyone with information is asked to report online via www.beds.police.uk or by calling 101, quoting reference 40/36217/22.
Alternatively, report information to Crimestoppers anonymously via 0800 555 111 or the Crimestoppers website at http://crimestoppers-uk.org
Crimestoppers, which is an independent charity, can offer rewards of up to £1,000 for information which can help lead to a conviction.