Mary eyed Tom's jacket where it hung on the coat rack at the other end of the office.

As had now become a daily custom at the breakfast table over the last month, this morning her father handed Tom yet another letter from his vexatiously persistent mystery correspondent. As usual, Tom simply slid it into his jacket pocket with just a word of thanks.

Mary watched the exchange silently, knowing to pass comment would only earn her the disapprobation of her papa, a disappointed sigh from her husband, and a tightened jaw from her brother-in-law. And yet despite telling Tom on the afternoon of her little emotional breakdown in the office two weeks ago that she accepted his letters were none of her business, she burned to know more.

Every morning, without fail, Mary felt anger and annoyance ratchet up in her chest as another letter arrived for Tom. Invariably, she found herself stewing on the situation at frequent intervals throughout every day, fretting that Miss Smith was inveigling her way deeper into Tom's life and – God forbid – his heart with every envelope that arrived at Downton Abbey.

This morning, events had proceeded along the usual lines – handover, thank you, pocket – except for the fact that Tom had spent every moment since then with her. They'd come directly to the office straight after breakfast, an early meeting with a supplier in the diary. Their schedule today meant he'd had no opportunity to either read his letter or stash it in his room as she supposed he usually did.

Which meant his latest letter was still in his pocket.

Mary glanced at the door, sensing an opportunity for sleuthing. Tom was currently outside with the supplier, who had arrived in a car which was making a rather unhealthy noise. Unable as ever to resist a mechanical puzzle, Tom had offered to take a look at the engine when they concluded their meeting. He'd taken his jacket off to avoid getting it dirty and left it hanging – completely unguarded – in the office.

Mary chewed her bottom lip, the temptation to rifle through his pockets and see if she could glean any useful information from the envelope nipping insistently at her. So far, she'd resisted, knowing it was wrong and underhand. But the urge to snoop was growing almost unbearable.

And Tom's jacket was right there, like a siren on a rock, enticing her to come and take a peek. Just one tiny little peek.

Finally, she could take it no more. She rose to her feet, circling her desk and walking over to the coat rack. She stood for a few seconds, staring at his jacket and debating the wisdom of what she was about to do. If Tom discovered her…

She cocked her ear, hearing his voice outside in conversation with Mr Spencer. If she was quick, she could probably get away with it. And it's not like she meant to read the letter.

She pushed her hand into the pocket she'd seen him slide the letter into and withdrew it. She stared at the envelope, then brought it to her nose, smelling a light fragrance coming from the stationery. Definitely from a woman then. Not that she'd ever really doubted that. But still, Mary felt her chest tighten with confirmation of that fact.

She examined the envelope, seeing Tom's name and their address written on the front in a feminine hand. Turning it over, she saw a return address neatly printed in the top left-hand corner, which she read over and over, committing it to memory.

'What do you think you're doing?'

Mary jumped, guiltily turning to face the door, the envelope still in her hands. Tom stood in the doorway, his sleeves rolled up, oil on his hands, an astonished look on his face.

'I… I was just…' she trailed off into silence, knowing he'd caught her red-handed and she had no defence.

Tom strode over to her and snatched his letter out of her hand, shoving it into his trouser pocket. He glared at Mary, his jaw tight with anger, then walked over to his desk, opening the cupboard behind it to retrieve his toolbox.

'I'm going back outside to fix Mr Spencer's car. We will discuss this when I return,' he said, banked fury in his voice. With that, he strode past her, barely able to look at her.

Mary closed her eyes, mentally kicking herself for being so stupid. She'd known it was a risk and yet she'd done it anyway. And now Tom was going to yell at her and the worst thing about it was that she knew he had every right to do so.

Tom came back into the office half an hour later. Mary had fair warning – she'd heard the car engine roar to life, purring again instead of coughing as it had done earlier, so he'd obviously worked his mechanical magic. Five minutes later, she'd heard Mr Spencer thank Tom profusely and drive away, tooting his horn as he went. She watched the door and braced herself for the scolding she was no doubt about to receive.

Tom didn't speak when he came back into the office. He stowed his toolbox neatly back in its cupboard without so much as looking at her. After that, he disappeared into the tiny kitchen next door and she heard the water running as he presumably washed the oil from his hands.

When he walked back in, rolling his sleeves down, Mary watched him silently, her eyes drawn to his strong forearms even as she waited for the storm of his anger to break over her.

He sat on the edge of his desk, legs crossed at the ankles, fastening his cuffs.

'So, do you want to tell me what you were doing going through my pockets?' he said, his voice surprisingly calm.

Mary saw no point in lying. 'I was curious about your letter.'

He was silent for a few seconds. 'Were you going to read it?'

'No! Of course not!' Mary said indignantly, despite knowing she could not claim any moral high ground in this matter. 'I would never do that!'

'Then what did you hope to achieve?'

Mary stared at him mutinously, hating every minute of this. 'I was looking to see if it had a return address on it.'

Tom nodded, pursing his lips. 'Did you recognise the address?'

'No. But I daresay I can find out who lives there.'

Tom sighed, closing his eyes briefly, before fixing his disappointed gaze on Mary again. 'I know you can. It's Lady Bagshaw's address.'

Mary pressed her lips together, swallowing down the bitterness she felt at finally knowing the truth about his mystery correspondent. 'So, I was right then. It's Miss Smith who is bombarding you with letters.'

'Well, it's not Lady Bagshaw. And I wouldn't call it bombarding me.'

'No? Then I'd hate to see what you would call bombarding when a letter every day for a month doesn't warrant that description,' Mary observed, acerbically.

Tom stared at her, a muscle in his jaw jumping as he ground his teeth together, trying to stay calm. 'Why are you so obsessed with my correspondence with Miss Smith?'

Mary didn't answer that, preferring to ask a question of her own. 'Are you having an affair with her?'

'That's none of your business.'

'I think it is.'

'And how do you come to that conclusion?'

'You're my niece's father.'

Tom stared at her incredulously. 'You're bringing Sybbie into this?'

'And you're my friend!' Mary cried in frustration, pushing up from her chair and stalking over to stand in front of him. 'Friends talk to each other, don't they? So, why won't you tell me anything?'

Tom sighed. 'Because this is nothing to do with you, Mary. It's between me and Lucy.'

'Lucy,' Mary scoffed, dismissively.

'Yes, Lucy,' Tom said, trying to hang onto his temper in the face of her rudeness.

'And there's a you and Lucy then, is there?' Mary asked, feeling decidedly sick at the thought.

'Possibly,' he said, shortly.

'Either there is or there isn't, Tom. She's making it perfectly obvious that she wants that to be the case. It's quite unseemly, really.'

Tom clenched his jaw again. 'It's all perfectly respectable if that's what you're worried about.'

Mary snorted, contemptuously. 'She's practically throwing herself at you! That's not what I call respectable!'

'She is not throwing herself at me. She's writing to me because we have a lot in common and she feels I can understand the position she's found herself in better than most,' Tom said, revealing more than he'd intended in an attempt to make her see how unreasonable her accusations were.

'But she barely knows you!' Mary exclaimed, incensed by the thought that Miss Smith could claim any common ground with Tom. 'What can you possibly have in common? Does she know how to fix a car? Does she know anything about estate management? Is she an expert in Irish politics?'

'There is the small matter of us both having been elevated into a different world after being in service,' he said, trying hard not to lose his patience with her.

'Oh, so that's what she's using as her leverage to get you to pay attention to her, is it?' Mary sneered. 'Her tale of rags to riches.'

'Why are you so against Lucy?' Tom snapped in exasperation. 'What has she ever done to you?'

'I just don't think she's right for you!' Mary declared.

'Why? Because she was a lady's maid? I was a chauffeur, Mary. I'm no better than her just because I'm a land agent and a car salesman now. And Lucy is not a lady's maid any longer.'

'It's got nothing to do with that!'

'Really? Are you sure you are being entirely honest about that?' he asked with obvious scepticism.

'It's nothing to do with her background! I just think you could do better than Miss Smith!'

'How? With whom? Which highborn lady do you think I should aspire to?' Tom challenged, pushing off his desk and getting in her space. 'Sybil was the only one of your lot with the gumption to follow her heart even if that meant marrying down as your type would put it. No other woman of rank would deign to give me a second look. I'll always be the chauffeur to them.'

'That's not true!' Mary protested, shaking her head fiercely. 'You're an attractive man. Plenty of women would give you more than a second look! And any woman would be lucky to have you, no matter how highborn she is!'

'As their bit of rough, maybe. Not as a serious suitor.'

'Don't do yourself down, Tom! I can't bear it when you talk about yourself like that.'

'But that's what I am to your people, Mary. A bit of Irish rough who doesn't know his place,' he carried on relentlessly, practically toe to toe with her now. 'Perhaps good enough for a tumble between the sheets for the more curious and daring amongst them, but certainly not good enough to take even to a local ribbon-cutting event never mind to escort them to society occasions. They couldn't possibly lower themselves to what they see as my level. Not outside the bedroom.'

Mary stared at him, at his face inches away from hers, his eyes blazing, and suddenly she couldn't stop herself. She surged forward and kissed him.

For a few seconds, he responded before making a noise deep in his throat and shoving her away from him.

'What are you doing?' he hissed, shock and confusion all over his face.

'I… I…' Mary trailed off, her face burning, completely at a loss as to what to say. All she knew was that she'd desperately wanted to kiss him and so she did.

'You can't… we can't…' Tom turned away from her, running a shaky hand through his hair. 'This... this never happened. It never happened.'

Mary watched him unconsciously mussing up his usually neat hair and her stomach clenched with pure want. That was the only way she could describe it and it floored her. Suddenly, she was completely aware that she wanted him with every fibre of her being.

'I think I should go,' Tom said, plucking his jacket from the rack, not looking at Mary or anywhere near her. 'I… yes, I'll go.'

Mary stood rooted to the spot, silently watching him, burning to touch him, to stop him leaving. But she was paralysed by her mind being stuck on one thought, blaring like a siren through her – she wanted Tom. She desired Tom.

He picked up his hat, took a few steps towards the door then halted, finally looking across the room at her, keeping a safe distance between them. He stared at her for a long moment, a series of emotions flashing across his face so quickly Mary couldn't even hope to read them. And then he turned and walked out of the office without another word, leaving her to collapse shakily onto the nearest chair and try to make sense of what she'd just done and her sudden, earth-shattering epiphany about her feelings for her brother-in-law.