A man armed with a metal object threatened staff at the BP garage on Dawlish Road near Matford on Sunday evening at about 20 past six.
He left when they refused to hand anything over.
DC Joanne Sargent said: “A man entered the station armed with a metal object. He demanded money from the shop assistant but left on foot when the assistant refused to hand anything over.
“The man is described as white, of a heavy build with black eyebrows, and was wearing a navy/dark blue jacket. He had his face covered by a dark scarf/balaclava.
“We are seeking witnesses to the incident."
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