THREE people arguing in the street vented their anger on a man walking his dog.
The 37-year-old victim walked past the group on School Lane in Dewsbury at 11.30pm last Tuesday.
He was approached by one of the men who punched him to the ground and carried on attacking him for no reason.
The other man and woman then joined in.
Another man arrived and helped him and police are keen to trace him.
He was in his 40s, stocky, about 6ft tall and was wearing a knee-length waterproof jacket.
The victim had suffered serious facial injuries and was taken to Dewsbury District Hospital, where he was diagnosed with a broken jaw.
He was transferred to Pinderfields Hospital in Wakefield for extensive treatment to the injury.
The thug who started the attack was described as white, in his late teens or early 20s, with short dark hair which was spiky at the front. He wore a dark waist-length zip-up jacket and dark jeans.
The other man and the woman were both white and also in their late teens or early 20s.
The woman was slim with blonde shoulder- length hair and wore a frilly blouse.
The good Samaritan, or anyone who knows who the three attackers are, should contact Pc Gavin Smith via Dewsbury police helpdesk on 01924 431059.