Cambodia is one of the world's poorest countries and notorious for child sex trafficking, making it a big destination for paedophiles and other sex tourists.
Geoff is sitting on a small, hard bed in a Cambodian brothel, his heart thumping fast.
He is 49 years old, a retired Australian diplomat with a wife and two grown-up children.
After a long, tense wait, a grinning teenaged boy opens the door and pushes in two young girls.
One says she is seven years old. The other is nine.
The younger one seems as nervous as Geoff, breathing heavily, as the boy explains exactly what she will do for $60.
Geoff sits back on the bed, a deliberately casual move, but it enables the top button on his shirt to point directly towards the girls' faces.
Hidden within that button is a tiny video camera and microphone.
Geoff, not his real name, is an undercover investigator wading through the depravity of Cambodia's paedophile industry.
Authorities are warning about the dangers of child sex tourism
He works for an international organisation dedicated to fighting injustice.
"The adrenalin is always pumping," he says, "no matter how many times you do it."
More often that not, the girls are drugged.
One of them described it to Geoff as feeling "like you're not really there." Some get an injection before each client.
Geoff works mainly in the capital, Phnom Penh, walking along grimy, jostling streets.
In the city centre there are plenty of brothels popular with so-called sex tourists.
Many of the girls in them are obviously under 18, the age of consent here, but their clients either do not realise, or do not care.
But out in the suburbs are places like Svay Pak.
For years this narrow clutter of bars and coffee shops has hidden what many believe was the world's top destination for paedophiles.
Geoff is well prepared.
He has his hidden camera, a can of pepper spray, a tracking device, and at least four assistants at close hand, ready to spring him if things turn ugly.
Many brothels are run by ruthless Vietnamese gangs.
Some are owned or protected by senior Cambodian police officers.
"The risks are real," says Geoff. "My wife was concerned to start with, but she's very supportive now."
Inside the brothel, Geoff is often trapped behind up to three sets of locked doors.
Only then are the children brought out and offered to him.
He talks, films, then uses one of half a dozen standard excuses to leave. "I'm just going to go and get a friend, and we'll be back soon."
Once he walked in on an elderly European man, raping an eight-year-old girl. "For like two seconds," he says, "I just couldn't move.
Sometimes the foreign tourists are only arrested when they get back to their own countries and confronted with Geoff's footage
"I remember seeing his clothes hanging on a peg. I guess it's lucky we're not allowed to be armed. I could have..." his voice trails off.
Instead, Geoff stuck to his undercover role, and closed the door.
Outside, a few minutes later, he alerted the police, but the man slipped out through a back door.
It can be frustrating, Geoff admits, but there is the compensation of knowing that as a direct result of his work, seven foreign paedophiles have been arrested in the past year-and-a-half.
And today Svay Pak is pretty much closed down, although Geoff knows that the children and their handlers will simply have moved on somewhere else.
Sometimes the foreign tourists are only arrested when they get back to their own countries and are confronted with Geoff's footage.
But the vast majority of cases involve Cambodians.
In February, Geoff gave evidence at a local trial in Phnom Penh. He co-operates closely, but secretly, with the Cambodian police.
The judge questioned three girls, aged 13 and 14, who had been rescued from a brothel.
They told their story, then asked the judge if they could stay and hear their abuser sentenced.
He got 15 years. "Not long enough," the girls told Geoff.
As you can imagine, Geoff's work is rarely straightforward. There is corruption in the police force and there are other complications and ethical dilemmas about his job.
Most diplomats have privately accused the foreign group of doing more harm than good
Say he asks a pimp to provide him with lots of girls or boys, is Geoff helping to rescue victims or could he be encouraging the brothel to go out and search for new children to corrupt?
A few months ago, another foreign group working to protect children from the sex industry organised a raid on a brothel.
It turned out the place was owned by a particularly powerful policeman.
A huge scandal followed and now everyone is jittery.
Most diplomats have privately accused the foreign group of doing more harm than good.
And Geoff complains that it now takes him many days, instead of hours, to get the police to authorise new raids.
But still he is busy.
When I called him yesterday he sounded elated. He had just finished another raid.
Three girls rescued, the youngest aged eleven. Two Vietnamese adults now in jail.
Geoff comes across as the solid, unflappable type. At weekends he plays rugby.
"After each operation," he says, "I need to take a couple of deep breaths. But there is no psychological damage, at least nothing now.
"Someone has to do this job. I guess it might as well be me."
From Our Own Correspondent was broadcast on Saturday, 11 June, 2005 at 1130 BST on BBC Radio 4. Please check the programme schedules for World Service transmission times.