The incident happened inside Mace Convenience Store in Broad Street at about 9.40pm last night.
The victim, a 14-year-old boy became involved in a verbal altercation with a group of men.
Following this altercation, the offenders assaulted the victim, stabbing him in his back and shoulder.
The victim is currently in hospital in a serious but stable condition.
Investigating officer, Detective Mark Wray said: "This is a shocking offence during which a 14-year-old boy has sustained two stab wounds.
"The offenders are described as being a group of Asian men that were all dressed in dark clothing.
"The victim stated that he was followed to the shop by a black four door car, which the offenders then used to make off from the scene. At this stage we do not have the make or model of this vehicle.
"I am appealing to anyone with any information, especially if you saw anything suspicious or a group of men or a black four door car in the vicinity of Broad Street last night.
"Anyone with information can contact police by calling 101 or visiting our website, quoting investigation reference number 43200012012."