A man suffered a fractured skull after he was seriously assaulted in Bridge Street, Northampton.
The attack happened between 1.35am and 1.45am on Thursday, July 28, when the lone victim was allegedly involved in an altercation with two other men.
Another man arrived on the scene and is believed to have run behind the lone man, punched him in the face and knocked him unconscious in the middle of the road.
The victim suffered a fracture to the back of his skull and needed 12 stitches for a cut lip.
The attacker is then believed to have run towards Gold Street with the two men who had originally been involved in an altercation with the victim.
These three men are all described as having been slim with short, dark hair. One wore a blue, denim-style shirt and blue denim jeans, another wore a grey hoody and dark slouch-style jeans, and the man who allegedly attacked the victim wore a red t-shirt, skinny dark jeans and black shoes.
Anyone with information about this crime can call Northamptonshire Police on 101 or Crimestoppers, anonymously, on 0800 111555.