The zombies are taking over the world and Big Dave thinks it’s hilarious. As a zombie man and his zombie dog wander down the dark alley searching for human flesh, I scoop handfuls of popcorn and shovel it into my mouth. Big Dave says he’s watched this movie many times and it always cracks him up.

“Boys, zombies, dogs, popcorn – an unbeatable combination,” says Big Dave, dropping the kernels down his throat.

These four things make me happy, which is a big deal since I’d been feeling properly fed up after the Dad incident two nights ago. Big Dave arrived at just the right moment with the movie and a popcorn bucket. I look over at him and he tells me to keep watching the screen because the best bit is coming up. He warns me to watch the zombie with the crazy eyes.

“That’s all of them,” I say and then I laugh.

I can’t help but like Big Dave. Despite knowing he has a wife and child, he’s fun to be around. No matter how many times I tell myself not to enjoy his company, I can’t do it. While he snorts at a zombie, I wonder if he hugs his wife the way he hugs Mum: all open-armed and smiling, then deep and comforting, with his fingers knotting her hair. I wonder if Caroline 1973 pats him on his bald head the way Mum does.

“There are more zombies now. They’re behind you,” shouts Big Dave. “The zombie dog wants a human bone. Look out! They’re getting into your house.”

“Where do you live?” I mumble through pillows of popcorn, but I keep my eyes on the TV screen.

Big Dave doesn’t look at me.

“Over in the Ireland estate.” He laughs and points at a zombie falling through a window before rolling down a flight of stairs.

“The houses are nice there,” I say.

“Yup, they are,” says Big Dave, adding, “but I’m only renting at the moment.”

“Have you been there long?” I ask.

“Not long. We’ve moved around a bit, I suppose.”

We, not I. Big Dave has just slipped up. Not that he’s noticed, because he’s too busy shouting at the man in the movie that there are zombies behind him. Big Dave says this bit gets him every time. “It’s ridiculous,” he says. “How can that bloke not know there are twenty zombies shuffling along behind his back? The mumbling and grunting should give it away.”

“Where’s your wife?” I don’t know why I suddenly blurt it out, but I do. It’s as though my tongue freaked out and said the exact thing I was thinking in my head.

Big Dave chokes on a bit of popcorn and bangs his chest with a fist. “Sorry, popcorn went the wrong way. It’s lethal stuff. I know of a woman who broke her back tooth because of the hard bits. Maybe I should give it up too before it gets stuck in my windpipe.” Big Dave gets up from the sofa and walks ridiculously fast towards the kitchen, saying he needs a beer before he pops his clogs from popcorn.

Big Dave is funny like that. Only I’m not laughing, because the zombies have left the house and made it to the shopping mall, and my big moment has gone. Gone but not forgotten. I’m already ahead on Operation Baskerville so, in theory, I could add another mystery to my list. This is the brightest of my bright ideas. I mean, how difficult could it be to find out more about Caroline 1973?

Operation Reichenbach is born.

Reichenbach Falls is where Sherlock Holmes takes on his arch-enemy, Moriarty. I read it in Dad’s Sherlock book and it was so exciting that a bit of chocolate wafer dropped from my mouth onto the page and now it acts as a brown bookmark. Of course, I’m not saying I’m likely to end up beside a waterfall, fighting tooth and nail. And I’m not saying Big Dave might be like Moriarty or that he’s toying with Mum’s affections, but if the deerstalker fits… Either way, if Caroline 1973 is out there, then I want to know all about her.

In a Sherlock mystery there are lots of clues. My first Operation Reichenbach clue is the one from Nina Biddolpho, who told me that Big Dave has a wife called Caz and a small son. Nina Biddolpho knows everything, what with her being a newsagent and gossiping to hundreds of people a day. You can hardly think you’re going to sneeze in Paradise without her offering to sell you a box of tissues.

I know Nina was absolutely certain about Big Dave’s secret because she got all excited and her eyes started twinkling (although that could have been the electric-blue eyeliner). And what’s more, Nina says she never tells lies. (But actually that’s probably a lie in itself, because everyone tells little lies from time to time. I’m always doing it. If Grace asks me if her bum looks big in her skinny jeans, I always say it looks tiny when I know it looks massive. However, if I say it looks massive, she turns into Ninja Grace and this is very bad for me.)

The second clue is Big Dave said we, not I. It is clear to me that he has a family. A person living on his own would not say we.

My third and final clue, and one that can never be hidden, is Caroline 1973 inked on Big Dave’s upper arm. Thanks to Grace leaving her gossip magazines on the living room table, I know that lots of stars get the names of their wives and children tattooed all over their body. And Big Dave can’t really deny the evidence when it’s written on his skin. (Suddenly, I catch myself wondering if Dad has the name Busty Babs tattooed on his body. Even worse, does he have the name of the beefy boy with the windmill arms too?)

Grace looks a bit surprised when I knock on her bedroom door, offering what’s left of the popcorn and asking if she wants in on Operation Reichenbach.

“Right ’n’ Back? It sounds like some American football team,” she says.

When I explain my mission, Ninja Grace nods her head and tells me it’s about time we joined forces to flush out the traitor in our midst. I try to say that this is investigative and not a witch hunt, but she says Big Dave is guilty until proven innocent. With her next breath, Grace says she wants to do something so utterly crazy that I’m speechless. That’s when she takes the opportunity to gulp down all the remaining popcorn. “You’ll come round to my way of thinking,” she says, wiping bits of popcorn from her lips. “You mark my words.”

But you can’t really. Mark words, that is. And no way is what she suggested ever going to happen, I think, as I’m in the bathroom swabbing the inside of my cheek with a cotton bud an hour later. What Grace suggested was stark raving bonkers on the madness scale. We’d be caught for sure, and that’s if I could even find out Big Dave’s address. I can’t get the directory out again when it took me an hour to clean up the hallway last time.

The cotton bud looks a bit slimy when I pull it from my mouth. I think there’s a bit of popcorn on it. You never see that happening on the TV. On the telly, when they take a swab for DNA, they never end up with chunks of someone’s movie-time treat. I had planned to put the cotton bud into a little plastic bag and keep it safe, because you never know when your DNA might be needed to prove that your dad is a big telly star. But what if there was a court case and I had to present this one as evidence:

Prosecution: M’lud. This boy’s DNA does not match Malcolm Maynard’s DNA.

Defence: Objection! I think you’ll find we are one hundred per cent certain it does. This is his son.

Prosecution: This swab only proves that the boy is half toffee-popcorn.

Defence: We think when you chew over the facts you will see we are correct.

I laugh and throw the popcorn-covered bud towards the bathroom bin. It misses and I sigh, then try to pick it up with my bare toes rather than my hands. When that doesn’t work I bend down and grab it and set it inside the bin beside an empty packet. Something makes me pull the packet out of the bin.

It’s a pregnancy testing kit.