I’ve moved through life, these past five years, barely remembering my own name. I’ve been living as Holly for five weeks. But there have been others, too. Some of them as young as seven, others, anywhere up to fifteen.

The scent of coffee drifts in from a Starbucks on the corner of New Globe Walk, overlooking the pier. I’ve parked my Ford Transit behind the wooden O of Shakespeare’s Globe. This is the only spot I can find without a double yellow line on the edge of London’s South Bank. I’m following this dirty trail to the end, to its rightful conclusion.

Last night, I was in South Kensington, next door to the tube, leaning against a lamp post beside a steaming falafel stand. The night before, in Willesden Green, loitering about a corridor in a dilapidated council estate. A week before that, in High Barnet, skirting the edge of a sports field in the rain.

Each week is different. My life is in a permanent state of standby. I’m ready to press play at the last minute, but sometimes, like this week, we’re called out three times. Other times, just once. It’s always the same, regardless. Everything runs like clockwork.

The Friday air is sweet and warm. The sky is tinged with a smear of burnt orange. The city streets of Southwark are swept cleaner than where I live in the leafy suburbs of Harrow-on-the-Hill, with its crooked cobbled lanes and posh public boys’ school; where you’d think the rich kids are protected from the horrors of predators, but sadly, not so.

I rest my elbows on the steering wheel. I bend my head to avoid being conspicuous. On the passenger seat to my left is the file I’ve collated of the back-and-forth in the forum, printed and bound in chronological order. Photos and videos are downloaded onto a USB stick. It’s taken five weeks to gather up this evidence, but it could all be for nothing unless the police do their job properly. It’s hard not to blame them for the state we find ourselves in, though I try, really, I try. Inaction is killing us all inside.

I’m tired, so very tired. A heaviness soaks into my skin. But I know the tiredness is only temporary. That eventually, when it’s all over and done with, I’ll feel euphoric.

There’s a rumble of a car engine behind me. I glance up at the wing mirror and see a Toyota Sedan, with blacked-out windows, pulling up. It has a dent just beneath the left headlight, which, in the warm haze of sunlight, appears like a giant blister. It’s Spider, short and stocky, with long black hair and a criss-cross tattoo like a collar around his neck. He’s in the driver’s seat wearing dark glasses, sporting his favourite combat jacket. His jaw is freakishly square, he’s chomping on gum.

Spike sits beside him, a black cap pulled down low over his face. I can barely make out his familiar hooked nose and ginger goatee, but I know it’s him by the way he hunches to the side, as if the weight in his body is unevenly spread. On nights like these, we move together as a three, like a pack of wolves at dusk, going only by our street names. We’ve carefully prepared for our sting, united by a strong, primal urge to protect the innocent. Spider torn by the pain of his own childhood, interrupted. Spike still searching for answers.

The engine switches off. Spike bends forward, as if fiddling with the car radio. Just then, my phone vibrates.

The headlights blink and Spider nods. It’s my cue to get out of the van. I wedge the file under my arm. The door thuds behind me. I head towards a wooden bench overlooking Bankside Pier and sit down, scanning the rippling water. A river bus passes, leaving foamy streaks on the surface. The sun disappears behind a cloud. I shiver in the cool breeze as the sky begins to darken. I’m frantically checking my phone, thumbing through the last messages I’ve received, or rather Holly has received, from Zia_123.

These are from Zia Ahmed, and each time, his messages are the same. But there are hundreds of men just like him, posting comments within only a few minutes of me uploading a new profile picture. This time, I did what I always do to prepare. I sat back, waiting for him to take the bait, careful not to coerce him. I chose my words carefully. But now I’m tested and tortured, every time, by the necessary levels of patience required. You wouldn’t believe how many of them there are out there. How little time it takes for a man to flash his penis to a child.

It’s 8 p.m. A bell tolls in Southwark Cathedral. I run things over in my mind, to be certain I’ve got everything with me. That I’ve left nothing out that might give the police an excuse to call out the integrity of what we’re doing. I dial the number for DCI Lawson. He picks up after the first ring.

‘We’re here,’ I say. ‘Zia’s on his way. I suggest you come now, if you want to be a part of this.’

I hear Lawson sigh on the other end of the line, a muffle as he calls out to someone in the background. There are footsteps behind him, the crackle of a car radio.

‘Don’t do anything stupid, Rita. Hold him there until we show up. I hope your evidence is solid. If not, it’ll be you who’s in trouble.’

My throat constricts, but I reassure myself that it doesn’t matter what he thinks. Our intention to help is genuine, backed by a legitimate citizen-led initiative that is very much needed.

I glance into the distance and see a stick figure walking towards the bridge, tall, wearing a hoodie with a white stripe down each arm. He looks left and right, then all around him. As he nears, I see his hood is up, strings pulled taut at the neck. His joggers are thin like drainpipes. He moves to the concrete edge and pulls out a phone from his pocket. I look down. Before me, a message appears:

I dial Zia’s number; I need to be certain I’ve got the right man. When I hear the phone ring, and see Zia place the phone to his ear, I hang up. I thumb a message:

From the corner of my eye, I see Spike and Spider walking, keeping close to the wall of the Moonlight café. I pick up speed, walking towards Zia, and as I do, Spike and Spider follow straight behind me; closing in.

As I near, Zia is still there, staring down at his phone. He doesn’t notice me, creeping up behind him – easy does it now. I start to run. Spike and Spider are running too. I try to catch my breath at the same time, I don’t want to alarm him. I can’t afford to mess things up at this crucial moment.

I press REC.

‘Hello, mate. It’s Zia_123, isn’t it? But your real name is Zia Ahmed?’ I slow down. ‘You’re here to meet Holly, is that right? Holly who’s twelve …’

Zia looks up. He scans behind him, then looks back at me. His face is blank, his eyes large and bulging. He has a moustache, thin and fuzzy. It makes him look young, like he might only be eighteen. He’s old enough to know better, however. He knows exactly what he’s doing.

‘You’ve asked her to meet you here, so that you can take her to a hotel, to have sex with her, isn’t that right, Zia? You’ve booked a room in the Travelodge. You’ve been sending her explicit messages, photos, videos of yourself, for the past five weeks …’

My words are rushed, I remind myself to slow down. The sentences must be clear and audible if they are to count.

Zia doesn’t answer. What can he possibly say? He steps back, shaking his head and hands. I see he’s about to run, but I know he won’t get far. Spike and Spider have him surrounded. Their eyes are fixed on him like prey.

Zia staggers back, falling to his knees. He’s unable to stop shaking.

I shout from above him, staring down at his crown. His hood slips off his head, falling in folds around his neck.

‘You’re here to meet Holly with the intention of having sex, aren’t you? You realise that’s child abuse? Holly is underage. She’s a minor.’

‘No, no!’

Here we go again.

It’s always the same with these men. First comes the denial, then the lies, the regret. Most often there is an apology, but rarely is there repentance.

Zia slaps his hands against his head. His fingers tug at his hair. ‘You are making a mistake. I only here for talking, for friendship.’...