The one-off special, which is set to air next month, will follow Pat and Mo in their early years. Emma Cooke will star as the 'younger' Pat and Lorraine Stanley will play the 'juvenile' Mo.
The storyline will reveal Pat and Mo's lives in the swinging 60's London and explain why the pair didn't speak for 30 years. Mo married Pat's brother Jimmy, starting a rift between the two.
The special episode will see the pair talking about their troubles in the present day Walford.