Murder detectives believe a paedophile could have been behind the disappearance of Lee Boxell more than 24 years ago.
In a Crimewatch appeal to be aired this evening, police will reveal they are working on the hypothesis the 15-year-old, who was last seen in Sutton High Street on September 10, 1988, may have been killed in Cheam.
A nationwide 25-year-search has produced no evidence that one of the UK's longest running missing persons is still alive so now, police are hunting for a body, and potentially his killer.
They are hoping tonight's Crimewatch, watched by 5m people, will give them answers.
The family of Lee, who would be 40 on Saturday, say this is the last chance to find out what happened to their son.
His dad, Peter Boxell, said: "We have done hundreds of appeals, but this is the most high profile. The police have thrown everything behind it and it's going to be our last chance to get answers and closure. Not knowing what happened to your child is the worst feeling."
On the show, Detective Chief Inspector Nick Scola will talk about paedophile activity in and around Cheam.
The Crimewatch reconstruction has been filmed in Sutton High Street, where Lee was last seen, and St Dunstan's Church in Cheam and an unofficial youth club called the shed.
Peter Boxell, said: "It nearly brought me to tears seeing Lee's actor, he looked a lot like him, even down to the ears. If someone saw him at the shed on that day, please tell police. We need to know what happened, or better still tell them where his body is so we can bury him and move on with our lives."
DI John McQuade, from the murder investigation team that is leading the investigation into Lee's disappearance, said the shed was a place where young people socialised.
He said: "The shed was a place where young people went to drink and smoke. Many of these people will now have children of their own, some may even be of Lee's age. If Lee is dead, we want to be able to return Lee's body to the Boxell family who have had 25 years of not knowing."
Lee Boxell was last seen in Sutton High Street near Asda at about 2.20pm on Saturday September 10, 1988, and police believe he may have walked to Cheam.
Last summer Metropolitan Police forensic teams spent three months using Army radar scanners to examine the graveyard at St Dunstan’s church.
They excavated parts of the graveyard looking for a body, but found nothing.
Crimewatch will be aired on BBC1 tonight at 9pm.