Police are appealing for witnesses after a man and woman were confronted by a man with a BB gun when they returned home on Saturday night.
The pair, both aged in their 20s, had gone back to their house on Monks Road where they found signs of a disturbance.
A man pointed a BB gun at them before fleeing the house and was subsequently chased by the male victim from Thomas Street to Cannon Street.
No-one was physically hurt during the incident but the victims have been left shaken up by the ordeal.
The offender is described as being around 5ft 7in in height and wearing a balaclava, camouflage jacket and dark blue jeans.
The BB gun has now been recovered by police who also believe that there were a number of people in the area at the time the offender ran from the house.
Investigating officers would like to hear from those people to see if they have any information that could assist the enquiry.
Anyone with any information is asked to contact Lincolnshire Police on 101 quoting incident 15 of November 1.
Alternatively contact the independent charity Crimestoppers anonymously on 0800 555 111.