I made a mistake. I know that now.

The only reason I did what I did was what I heard on that train. And I ask you, in all truthfulness – how would you have felt?

Until that moment, I had never considered myself prudish. Or naive. OK, OK, so I had a pretty conventional – some might say sheltered – upbringing but . . . Heavens. Look at me now. I’ve lived a bit. Learned a lot. Pretty average, I would argue, on the Richter scale of moral behaviour, which is why what I heard so shook me.

I thought they were nice girls, you see.

Of course, I really shouldn’t listen in on other people’s conversations. But it’s impossible not to on public transport, don’t you find? So many barking into their mobile phones while everyone else ramps up the volume to compete. To be heard.

On reflection, I would probably not have become so sucked in had my book been better, but to my eternal regret I bought the book for the same reason I bought the magazine with wind turbines on the cover.

I read somewhere that by your forties you are supposed to care more about what you think of others than what they think of you – so why is it I am still waiting for this to kick in?

If you want to buy Hello! magazine, just buy it, Ella. What does it matter what the bored student on the cash desk thinks?

But no. I pick the obscure environmental magazine and the worthy biography, so that by the time the two young men get on with their black plastic bin bags at Exeter, I am bored to my very bones.

A question for you now.

What would you think if you saw two men board a train, each holding a black bin bag – contents unknown? For myself, the mother of a teenage son whose bedroom is subject to a health and safety order, I merely think, Typical. Couldn’t even find a holdall, lads?

They are loud and boisterous, skylarking in the way that so many men in their twenties do – only just making the train, with the plumped-up platform guard blowing his whistle in furious disapproval.

After messing about with the automatic door – open, shut, open, shut – which they inevitably find hilarious beyond the facts, they settle into the seats nearest the luggage racks. But then, apparently spotting the two girls from Cornwall, they glance knowingly at each other and head further down the carriage to the seats directly behind them.

I smile to myself. See, I’m no killjoy. I was young once.

I watch the girls go all quiet and shy, one widening her eyes at her friend – and yes, one of the men is especially striking, like a model or a member of a boy band. And it all reminds me of that very particular feeling in your tummy.

You know.

So I am not at all surprised or in the least bit disapproving when the men stand up and the good-looking one then leans over the top of the dividing seats, wondering if he might fetch the girls something from the buffet, ‘. . . seeing as I’m going?’

Next there are name swaps and quite a bit of giggling, and the dance begins.

Two coffees and four lagers later, the young men have joined the girls – all seated near enough for me to follow the full conversation.

I know, I know. I really shouldn’t be listening, but we’ve been over this.

I’m bored, remember. They’re loud.

So then. The girls repeat what I have already gleaned from their earlier gossiping. This trip to London is their first solo visit to the capital – a gift from their parents to celebrate the end of GCSEs. They are booked into a budget hotel, have tickets for Les Misérables and have never been this excited.

‘You kidding me? You really never been to London on your own before?’ Karl, the boy-band lookalike, is amazed. ‘Can be a tricky place, you know, girls. London. You need to watch yourselves. Taxi not tube when you get out of the theatre. You hear me?’

I am liking Karl now. He is recommending shops and market stalls – also a club where he says they will be safe if they fancy some decent music and dancing after the show. He is writing down the name on a piece of paper for them. Knows the bouncer. ‘Mention my name, OK?’

And then Anna, the taller of the two friends from Cornwall, is wondering about the black bags and I am secretly delighted that she has asked, for I am curious also, smiling in anticipation of the teasing. Boys. So disorganised. What are you like, eh?

But no.

The two young men have just got out of prison. The black bags contain their personal effects.

I can actually hear myself swallowing then – a rush of fluid suddenly filling the back of my throat and my pulse now unwelcome percussion in my

ear.

The pause button is pressed, but not for long enough. Much too quickly,

the girls are regrouping. ‘You having us on?’

No. The boys are not having them on. They have decided to be straight with people. Have made their mistakes and paid their dues but refuse to be ashamed.

Cards on the table, girls? Karl has served a sentence at Exeter prison for assault; Antony for theft. Karl was merely sticking up for a friend, you understand, and – hand on heart – would do the same again. His friend was being picked on in a bar and he hates bullying.

Me, I am struggling with the paradox – bullying versus assault, and do we really lock people up for minor altercations? – but the girls seem fascinated, and in their sweet and liberal naivety are saying that loyalty is a good thing and they had a bloke from prison who came into their school once and told them how he had completely turned his life around after serving time over drugs. Covered in tattoos, he was. Covered.

‘Wow. Jail. So what was that really like?’ It is at this point I consider my role.

Privately I am picturing Anna’s mother toasting her bottom by her Aga, worrying with her husband if their little girl will be all right, and he is telling her not to fuss so. They are growing up fast. Sensible girls. They will be fine, love.

And I am thinking that they are not fine at all. For Karl is now thinking that the safest thing for the girls would be to have someone who knows London well chaperoning them during their visit.

Karl and Antony are going to stay with friends in Vauxhall and fancy a big night to celebrate their release. How about they meet the girls after the theatre and try the club together?

This is when I decide that I need to phone the girls’ parents. They have named their hamlet. Anna lives on a farm. It’s not rocket science. I can phone the post office or local pub; how many farms can there be?

But now Anna isn’t sure at all. No. They should probably have an early night so they can hit the shops tomorrow morning. They have this plan, see, to go to Liberty’s first thing because Sarah is determined to try on something by Stella McCartney and get a picture on her phone.

Good girl, I am thinking. Sensible girl. Spare me the intervention, Anna. But there is a complication, for Sarah seems suddenly to have taken a shine to Antony. There is a second trip to the buffet and they swap seats on their return – Anna now sitting with Karl and Sarah with Antony, who is telling her about his regrets at stuffing up his life. He only turned to crime out of desperation, he says, because he couldn’t get a job. Couldn’t support his son.

Son?

It sweeps over me, then. The shadow from the thatched canopy of my chocolate-box life – me shrinking smaller and smaller into the shade as Antony explains that he is fighting his ex for access, telling Sarah that there is no way he is going to have his son growing up not knowing his dad. ‘Don’t you think that would be just terrible, Sarah? For him to grow up not knowing his dad?’

Sarah is the one who is surprising me now – there’s a catch in her throat as she says she thinks it’s really cool that he cares so very much, because many young men wouldn’t, would just walk away from the responsibility. ‘I feel really awful now. Us banging on about Stella McCartney.’

And the truth? At this point I have absolutely no idea about any of it anymore. What do I know? A woman whose son’s only access battle involved an 18-certificate film at the local cinema.

An hour of whispering follows and I try very hard to read again, to take in the pluses of the quieter generation of wind turbines, but then Antony and Sarah are off to the buffet again. More lager, I am thinking. Big mistake, Sarah. And this is when I decide.

Yes. I will head to the buffet myself on the pretext of needing coffee, and in the queue or passing in the corridor will feign trouble with my phone. I will ask Sarah for help – hoping to separate her from Antony for a quiet word – and give a little warning that she needs to step away from this nonsense or I will be phoning her parents. Immediately, you understand me, Sarah? I can find out their number.

Our carriage is three away from the buffet. I stumble into seats passing through the second, bump-bump-bumping my thighs, and then feel for my phone in the pocket of my jacket as I pass through the automatic doors into the connecting space.

And that’s when I hear them.

No shame. No attempt even to keep themselves quiet about it. Making out, loud and proud, in the train toilet. Rutting in the cubicle like a pair of animals.

I know it’s them from what he’s saying. How long it’s been. How grateful he is. ‘Sarah, oh Sarah . . .’

And yes, I admit it. I am completely shocked to the core of my very being. Hot with humiliation. Furious. Winded and desperate, more than anything on this planet, to escape the noise.

Also the shame of my naivety. My ridiculous assumptions.

I stumble across the corridor to the next set of automatic doors and into the carriage, breathless and flustered in the scramble to put distance between myself and the evidence of my miscalculation.

Nice girls?

In the buffet queue, I am listening again to the pulse in my ear as I

wonder if someone else will have heard them by now. Even reported them?

And then I am thinking, Report them? Report them to whom, Ella? Will you just listen to yourself? Other people will do precisely what you should have done from the off. They will mind their own.

At which point my emotions begin to change and I am wondering instead how I came to be this out of touch, this buttoned up. This woman who evidently has not the first clue about young people. Or anything much.

Into my head now – a kaleidoscope of memories. Pictures torn around the edges. The magazines we found in our son’s room. That night after the cinema when we came home early to find Luke trying to override the Sky security to watch porn.

So that on this wretched train, I find that I need very urgently to speak to my husband. To my Tony. To reset my compass.

I need to ask him if the whole problem here is not with them but with me. Am I altogether ridiculous, Tony? No, really – I need you to be honest with me. When we had that row over the Sky channels and Luke’s magazines.

Am I the most terrible prude? Am I?

I do try to ring him, actually – that night from the hotel after the conference session. I want to tell him how I did the sensible thing and moved to the other end of the train. Minded my own. The girls clearly quite streetwise enough.

But he is out and hasn’t taken his mobile, being one of the few who still thinks they give you brain cancer, and so I speak instead to Luke and find that it calms me to hear him describe supper – a tagine from a recipe he downloaded on a new app. He loves to cook, my Luke, and I am teasing him about the state of the kitchen, betting he has used every appliance and pan on the property.

Then it is the morning in the hotel.

I so hate this sensation – that out-of-body numbness born of air conditioning, a foreign bed and lack of discipline over the minibar. My hotel treat – a brandy or two after a long day.

It is barely six thirty and I long for more sleep. Ten futile minutes and I give up, eyeing the sachets of sadness in the little bowl alongside the kettle. I always do this in hotel rooms. Kid myself that I will drink instant coffee just this once, only to pour it down the bathroom sink.

I stare at the line of empty miniatures, wincing as a terrible thought flutters into the room. I glance at the phone by the bed and feel a punch of dread, the familiar frisson of fear that I have done something embarrassing, something I am going to regret.

I turn back to the row of bottles and remember that after the second brandy last night, I decided to phone directory enquiries to track down the girls’ parents. I go cold momentarily at the thought of this, my memory still

hazy. Did you actually ring? Think, Ella, think.

I stare again at the phone and concentrate hard. Ah, yes. I am remembering now, my shoulders relaxing as I finally see it. I was holding the phone and then at the very point of dialling, I realised that I wasn’t thinking straight, and not just because of the brandy. My motivation was skewed. I wanted to phone not because I was worried for the girls, but as a punishment, because I was angry at how Sarah had made me feel.

And so I did the sensible thing. I put the phone back down, I turned out the light and I went to sleep.

Good. This is very good. The relief now so overwhelming that I decide by way of celebration that I will try the instant coffee after all.

I flick on the kettle first and then the television. And that is when it comes. The single moment – suspended at first and then stretching, stretching, beyond this room, beyond this city. The moment in time in which I realise my life is never going to be the same again.

Not ever.

The sound is muted from the late-night film I watched with the subtitles on to spare disturbing the guests next door.

But the picture is unmistakable. Beautiful. A photograph from her Facebook page. Her green eyes glowing and her blonde hair cascading down her back. She is at the beach; I recognise St Michael’s Mount behind her.

And somehow my body has zoomed backwards – through the pillow and the bedstead and the wall – until I am watching the screen from much further away. This screen that is scrolling putrid, awful words: Missing . . . Anna . . . Missing . . . Anna . . . The kettle screaming angry clouds onto the mirror while I am planning the calls in my head all at once.

A black and terrible jumble of excuses. None of them good enough. To the police. To Tony.

You have to understand that I was going to phone . . .