My wife strapped in Freddy, shut the door and turned towards me. "I have a 10 o'clock meeting with Patrick Beeman. To discuss what needs to be done." I suppose I nodded. Freddy wanted me to do my Flesh-Eating Zombie thing, but after Lorna's declaration, it was beyond me. She said something else that I heard but didn't really hear, climbed into the car and drove off. Freddy looked through the back window: his puzzlement gave me the strength to muster a weak smile and a wave. Lorna's parting words drew into focus: "You begin too many sentences with the word 'I'." I stayed on the drive, contemplating the sequence of spiteful dares that led to this point of no return, until the sky began gobbing warm rain, faster and thicker. Upstairs, in my office, lists were waiting – lists of lists – oh, Jesus, the paperwork ahead! I couldn't face it, and ran around the side of the house, down the garden and into the shed. On a half-empty tub of weatherproof paint, I sat out a berserking shower. I'd never Do Anything Stupid – how could I scar Freddy? – but the thought of 10 packets of Nurofen Express waiting in the back of my bedside cupboard was a reassuring one.

When Freddy's scarecrow was dripping dry in the sun, I found a mismatched pair of gardening gloves and a trowel, and attacked the knotweed, nettles and docks choking the veggie patch. I dug it last year during the banking crisis, when the age of supermarkets was looking shaky. Who'll end up eating these courgettes, spuds and onions now? Our back garden is long, thin and mature-suburban, with a copper beech at the end. Lorna says trees have no business being dark, but its bruised maroons and shimmering there nows always steady my head. Freddy wants a tree-house, and I always meant to build one in the copper beech. Nothing fancy – just a sort of tree-borne raft, like the one me and David Ockeridge built in his dad's orchard, when we were kids in Black Swan Green. In my daydream I'd tell Fred that the copper beech is guarding his prezzie, and I'd watch him run down, find the rope ladder, and climb up to a better, leaf-shushed world. The thought of another man building Freddy's tree-house in an ordinary green beech made me hurl my trowel through the shed window. Shark-fins of crashing glass followed the trowel. After an immobile minute I shouted, "You stupid – effing – idiot!" Now I'd have to invent one of my one-legged excuses. Freddy still believes them, probably, but Lorna would think, Yeah, right. I have to be more careful. The Biro of Patrick Beeman will hover over his legal pad: "How does your husband's passive-aggression tend to manifest itself, Mrs Briar?" And she'd reply, "I prefer to be called by my maiden name, from now on."

The weeds on the compost heap, I began feeding the shrubs with Blood, Fish and Bone. Three miles away Patrick Beeman would be working down a list of bullet points with Lorna: General Procedure, Custody, Money, House, Pensions, Miscellaneous, Standard Fees. My mother is going to crumple up: we've kept The Troubles well hidden. "Whose idea was it first?" she'll want to know, meaning, "Whose fault is it?" There's no Somebody Else on Lorna's side, I'm sure – she's too miserable to be in love – and God knows I've been too busy trying to keep my alleged ­ consultancy afloat to think about illicit sex, beyond the YouTube variety. Though sure, there's no denying that the money stuff hasn't helped the marital stuff. "When you're a guy, and a dad," I told Freddy's scarecrow, "and you have to ask your wife to put £5,000 of her bonus into the joint account so that the garage won't refuse your card, and all the jokes about being a Kept Man are worn away, the word is 'vasectomising'..."

Garden time swallowed me up, because the next thing I knew, Lorna was calling me from the patio. It felt like a humid Friday noon-time because it was. This patio scene contained a number of abnormalities, however, like one half of a Spot-the-Difference puzzle. First, Lorna was using my name, insistently and repeatedly: since The Troubles kicked off, she has become adept at framing sentences in which I don't exist. Second, she was actually maintaining eye contact: she usually addresses the room, or the air around me. Third, she was taking the trouble to announce her return instead of leaving it to doors, kettles and cisterns. Fourth – and this shook me most of all – Ms I-Will-Survive looked alarmed and needy. In an instant, a prayer was hurtling its way to the headmasterly God of my C of E childhood: "Please please please, don't let anything have happened to Fred."

"Nick, there's a rat trapped behind the fireplace!"

Like Captain Sensible a lifetime ago, I said, "What?"

She said, "A rat's trapped behind the fireplace."

I neglected to thank God for preserving Freddy.

She didn't think I'd heard. "A rat is trapped behind the fireplace."

It's not a rat, it's a mouse, I thought; and probably not even a mouse.

Lorna has a thing about mice, even imaginary ones.

I said, "You mean, in the fireplace? In the glassed-in box bit?"

She said, "No, I mean behind the fireplace: as in 'behind'?"

She does that Australian last-syllable lift just to annoy me.

"What makes you think that?"

"Well for one thing I can hear it."

The Devil made me say, "Why don't you phone Patrick Beeman?"

We are learning a lot about contempt, Lorna and me.

"Fine," she said, eventually. "Fine. Thanks so much."

I found myself walking up the garden.

She asked, as I passed, "What happened to the shed window?"

"Junk-slide: the handle of the mower fell through it."

Me and Lorna have sort of Berlinned the house into her zone and mine. Freddy passes through the checkpoints without noticing them, but the living room is deep in her territory: it's been weeks since I set foot there. As we knelt before the stove's glass front, something inside – and, yes, behind it – sensed us. Invisible claws sprinted up and slid down the steeply-sloping cast iron. I had to admit – though I didn't, yet – that no mere mouse could make such a hefty scrat-scrat-scratting.

Back when we bought the house, back when unborn Freddy was the size and shape of a cashew nut, the hearth was covered with cardboard and brown tape. Dreaming of chestnuts on open fires, we stripped it off to discover that the chimney acted as a vertical wind tunnel that sucked the heat out of the house. Our Estonian builder suggested a box-stove with a glass door that backed into the pre-existing hearth, and a fake marble surround to seal it in. We agreed, but the stove, when it arrived, had only half the depth of the fireplace. This would create a gap between the back of the stove and the bricks of the hearth, but with Rumsfeldian clarity our builder assured us, If a thing can get in, a thing can get out.

Except, plainly, rats.

Lorna wondered aloud whether it might be a bird.

"Not unless it can unscrew crow-guards off chimneys."

"A rat, then." She glared at me, sweatily. "Like I said."

"I wonder how long it's been in there."

"Fred was watching a DVD yesterday. There was nothing then."

24 hours, then, I thought.

She asked, "Is there anyone we could phone?"

"The RSPCA? They'd do a golden eagle or sea otter, but 'Thank God you answered! There's a rat stuck behind our fireplace – dispatch Team Rodent Rescue before it's too late!'"

"I meant a vermin controller." She added a basilisk-voiced, "Nick."

"One stupid rat hardly counts as an infestation."

"Can you pull the surround off, and... sort of open it up?"

The rat heard and detonated a volley of heavy claws.

"Not without slicing through the silicone sealant."

I said these words like a poor impersonator of a CID officer.

She said, "We could just leave it, and hope it finds its way out..."

"That's a non-solution. It'll get weaker and weaker."

The rat climbed and slid and climbed and slid.

Lorna swallowed. "This'll give Fred nightmares, you know. Remember the Doctor Who Incident."

As if she'd let me forget. "What worries me is the stench. If Patrick Beeman advised you to sell the house, it'll cost you a few thousand."

"Suppose we address one shitty problem at a time?"

"Course, we could light the fire and... sort of... cook it."

Wishing I hadn't said that, I tested the sealant between the stove's front frame and the stone surround with my thumbnail.

Lorna said, "Annette's bringing Fred back by two."

The mantelpiece clock – a wedding gift – read 12.35.

"I'll have to pull out the stove, somehow. I'll go to the hardware store before they close for lunch: I need to buy some gear. I hope you don't mind all these sentences beginning with 'I', by the way?"

The strip of shops is only a quarter hour by foot alongside the railway line, and across a park with a slimy pond, but the lack of time called for the car. There was a thing on the news about a divorced dad in Bristol denied the right to see his young sons. He lost it, and shot the family, finishing with himself. God knows I'm not saying it's justifiable, ever: or that I would, or could, bludgeon my own soon-to-be-ex-wife to death; much less little Fred, as he lay asleep, before knocking back 120 Nurofen Expresses with a bottle of Kilmagoon. No. All I'm saying is that our own Troubles help me understand why the man in Bristol did it. That's all.

Waiting at a zebra crossing for an old woman I had one of my Tourette's Moments. It's not real Tourette's Syndrome – it's a spasm of remembered shame – but it made me whack my knuckles against my temples and shriek like a Bee Gees harpy, "I'm so sorry!" To my mortification, the old woman heard: she stopped and stared at me like I was an utter bloody nutter. At the next set of traffic lights I had another Tourette's Moment about the last one and roared, "Damn you to Hell and back, Nicholas Briar!" How blissful, how liberating, a bullet would be. Not that I'd ever do it.

Certainly, 1-Stop Hardware is a place where I could die happy. The smell pacifies me better than any of Lorna's £30-a-bottle aromatherapy gunk: the iron lard of machine-grease; the masculine whiff of pet products; the pungency of freshly-moulded Chinese plastic. The rows and columns of screws and hooks in brass and steel, graded according to thickness and length, are a visual sedative. There is nothing in this world, avow the displays of tools, so knackered and broken that we cannot sand it down and fix it up. The names of paints are soul-watering: "Santorini Dusk", "First Tomato" or "Martha's Vineyard". I enjoyed the roguish camaraderie of carpenters and electricians who grabbed supplies for their afternoon jobs and bantered with Robbo and the Jonester at the till. Nobody would mistake me for a seasoned professional, but so long as I keep my mouth shut who's to tell I'm not a competent amateur? I gathered three types of knives; a fresh tube of Silico-4-Pro; a plunger to apply it; a wide, flat chisel; a pincher-clampy thing; and a pair of liver-coloured resin gloves. I paid, put my purchases in a box, and left 1-Stop taking a last backward glance, which is why I failed to notice the BlackBerry-wielding meteor hurtling down the pavement until it was too late.

Patrick Beeman checked his BlackBerry wasn't scratched before turning to the man on the pavement. He laughed with dismaying sincerity when he recognised me. "Enjoy the trip?" He extended his muscular arm to help me up and my treacherous hand obeyed. "No permanent damage, I trust?"

Lorna is leaving me, I thought, and my coccyx is mashed.

I brushed myself down. "Never better. I do hope you're well."

"Oh, mustn't grumble. No, mustn't grumble."

"I trust you had a constructive meeting with my – with – Lorna?"

Patrick Beeman made a sympathetic hum. "Sorry about the way things have turned out, but hey-ho. One addresses the details and moves on. Speaking of which, Nick – I can call you Nick, right? – you must sort out your legal representation ASAP." He pronounced this "ace-app". He watched me gather my spilt purchases. "Spot of DIY, is it?"

"There's a rat trapped behind our fireplace."

He made a That's Weird face. "'Behind'? Not 'in'?"

"It's finicky to explain. It's one of those slotted-in-and-sealed jobs."

"And you really think it's a good idea to open it up yourself?"

"It's either that, or live with the stench of a putrefying rat."

"Well, hey." Patrick Beeman shrugged. "I grew up on the Isle of Man. Wear a face mask, though, for Christ's sake. Put yourself in starving, hacked-off Mr Ratty's skin: what's the first thing he sees as light floods his sunless prison cell? Juicy eyeballs and gristly nose: dinner. Think American Psycho. Hey-ho. Have your solicitor, whomsoever you appoint, phone me ace-app so we can get the ball rolling. You'll want to change your wills, too, right? I'd rather be dragged over burning coals than smear any colleagues, but the McWhinney woman over Superdrug is a Pollyanna Coke-Head; and I'd not trust Mr Timothy Finn and Associates with a stapler, let alone access to my only son. Cheery-bye for now."

Lorna was slicing tomato and basil – we never eat together – but she came to the living room, to avert charges of Not-My-Problemism. "Getting the rat out from the fireplace is one thing," I told her, unpacking my box of gear, "but getting it out of the house is another."

Lorna said, "We should make a rat-run, leading to the front door."

I said, "Obviously. Obviously."

Two rolled-up hallway carpets made a single-lane highway to the propped-open front door. Lorna made a canyon out of the sofa and armchairs leading from the fireplace to the hallway. I remembered decorating the house with warm, funny, pregnant Lorna.

I opened the window as wide as it would go.

Lorna asked, "What's that in aid of?"

The truthful answer would have been, "To postpone a rat-bite for about 10 more seconds." I replied, "A mature black rat can leap as high as you. There's a chance it'll see the window and just go for it." Classic Nick Briar bullshit: Lorna provokes it out of me, like a laxative.

The blade slid through the sealant. Lorna was watching from across the room, gripping a tennis racket like a vampire-huntress gripping her wooden stake. Upon hearing the knife, the rat flung itself around like a madman. Around then, I realised that the exercise was pointless: the sealant wasn't glue, but just sealant to stop smoke. The frame of the box-stove was probably bolted to the floor through its feet, and the surround was immovable unless we ripped out the entire fireplace. Not yet wanting to admit this to Lorna, I carried on until the middle of the top edge. Words were embossed on a raised flap. "TO OPEN FRONTAGE, CLASP AND PULL."

I carried on with my knife around the entire frame.

"Right. That ought to do it."

I put on my goggles, clasped the raised flap and squeezed.

Lorna asked, "There's an opening mechanism?"

There was resistance, but a spring-loaded latch clunked.

The rat responded with a cold-blooded rat-a-tat-tat.

"Of course," I said. "But I had to unseal the silicone surround first." I put on my resin gloves. "Well... nice knowing you, Lorna." I pulled. The box section of the stove tipped forwards, which opened a gap of three inches along the top edge. It wouldn't open any further.

My pulse twanged fast and rubbery.

Cautious taps, without backslides, climbed up a shallower incline.

I moved back, gripping the other tennis racket.

A blurred brown paw, as big as a dog's, half-appeared.

Jesus on the Crapper, I thought, it must be massive...

The paw probed forwards, drawn by the light. It had one sharp claw, as sharp as a bird's beak: because this is what it was.

A dazed wren was perched on the top edge of the stove.

Perhaps it tried to understand the room and its wrongnesses.

But then it launched itself straight at the window...

...where it would brain itself on the glass and snap its neck.

But through the open window and on, it plunged into the afternoon...

...into the crisscrossing, traffic-drowsy, leaf-shushed afternoon.

Looking at my wife's face, I wished that time would stop.