As far as Mary was concerned, the next few weeks were hell. She saw less of Tom than she had since his months in America a couple of years ago. They still had meals together with the rest of the family, they still both worked on estate business, but they were never alone.

She missed him dreadfully. It was as simple as that. She missed talking things through with him about the business, bouncing ideas off each other, and sharing decisions. Unless they had a meeting with someone else present, they exchanged information through notes left on each other's desks.

She also missed talking to him about everyday things: about the children's exploits, chatting about acquaintances and the rest of the family, generally sharing the minutiae of their lives. They would make small talk with each other when the family was present to try to dispel any suspicion, but it wasn't the same. This whole situation highlighted the fact that outside of work and her newfound attraction to him, Tom was her best friend. Mary had never been much of a woman's woman. In her experience, women saw her as a threat rather than a friend or an ally and so – with the exception of her grandmother, Sybil and Anna – she had always preferred the company of men. Now, with Tom largely removed from her life in one fell swoop, she was lonely.

She tried her best not to think about him in any way other than as her brother-in-law, but it was almost impossible. Every time she saw him, her breath caught in her throat and her eyes lingered on him. Her heart raced and her pulse hammered. Butterflies plagued her whenever he was near. There was no let-up in any of that, despite her best efforts. When he wasn't around, she could lose time thinking about the way the corners of his mouth creased when he smiled, the way he moved his hands when he talked of something he was passionate about, the way his eyes twinkled when he was amused by something she said. She missed his voice, that gentle Irish lilt that soothed her so much. No matter how she tried, she couldn't banish him from her mind.

And then there was Henry. After realising her feelings for Tom were no longer platonic, she'd forced herself to think long and hard about her relationship with her husband. She couldn't help but think if things were perfect with Henry, she would never have felt drawn to Tom.

Looking back, she could see how rushed their courtship had been and she remembered the doubts she'd had about it. Everyone else had been so sure they were right for each other – not least Tom, she thought wryly – and Mary couldn't deny that Henry had been exciting, dynamic, different from the other men who had tried to capture her heart after she lost Matthew.

She remembered the panic she'd felt the day Henry had left Downton without a word to her after he'd surprised her and she'd rejected him. The raw feeling of despair that she may have sabotaged her only realistic chance of happiness. That feeling had led her to lash out viciously and derail Edith's relationship with Bertie by revealing the truth about Marigold. And it had also prompted her to listen to Tom and her grandmother when they talked about how Henry was the right man for her. And so she'd summoned him back to Downton and they'd been married within the week using the special licence Henry had finagled from his bishop uncle. It had been a whirlwind and she'd been caught up in it, propelled by the memory of that awful soul-deep despair. She hadn't stopped to give any further consideration to her doubts or the fact that she was committing herself for the rest of her life to a man she barely knew.

Now, almost two years into their marriage, she could see how they weren't a perfect match, not even close. They had very little in common, were in many ways chalk and cheese as far as their interests went. Although he'd given up motor racing, Henry was still consumed by the world of motor cars, spending much of his time on the road at motor shows or securing deals with manufacturers for Talbot & Branson Motors. While she wished him and Tom every success in their venture, she had no desire to spend hours listening to Henry wax lyrical about motor cars. Likewise, he had no interest in Downton and the struggles she and Tom faced in keeping the estate both afloat and intact in this modern world. In many ways, she thought, it was an ironic twist of fate that Tom was the bridge between their two worlds.

When Henry was at home, they rubbed along well enough together, but she'd come to the conclusion that while she was fond of him, she didn't feel any deep love for him. She didn't particularly miss him when he was on his travels except for on the rare occasions when she had to attend events alone. But even then, Tom often stepped in to escort her in her husband's absence.

The physical side of her relationship with Henry had always been satisfactory, which had been a great relief after her disastrous dalliance with Tony Gillingham. When Henry was at home, he made it perfectly clear that he found her desirable, a feeling she always enjoyed. Now, however, when Henry touched her, she had to force herself to stay in the moment, to not slip into a fantasy of Tom. She wasn't always successful in that, and the guilt when the fantasy made the reality better tormented her and made her increasingly turn away from Henry in the bedroom.

And Tom was right, she reflected glumly. That final consensual kiss she'd talked him into in the office was a mistake. She'd been a fool to think it would just be a delicious memory, a way to mark the end of a short, inexplicable period of infatuation with her brother-in-law. In that kiss, Mary had tasted temptation and she wanted more.

But she couldn't have more. Instead, she had less. Less contentment with her marriage, less peace of mind, less Tom in her life.

She couldn't help but feel like the whole situation was slowly killing her, sucking all the lightness and brightness out of her life. She felt herself becoming tetchier, sadder, more withdrawn.

And one by one, the family slowly noticed that she and Tom were no longer the tight unit they used to be.

'Have you upset Tom again?' Henry asked as they readied themselves for bed just over a week after her deal with Tom.

'No, of course not. Why do you ask?' Mary said, determinedly avoiding his gaze as she rubbed in her face cream.

'Something seems off between the two of you. You barely seem to speak to each other these days, not like you used to anyway.'

'I hardly think that's true.'

'No? This evening at dinner, you hardly exchanged a word beyond the niceties,' Henry said, climbing into bed and settling himself down. 'Usually, when you two are together, I struggle to get a word in edgeways.'

'I don't know what you want me to say, Henry. Tom and I are not fighting,' Mary said, keeping her eyes firmly on her reflection. 'I thought you'd be pleased about that.'

'So, you've put this business about Miss Smith's letters to bed then?'

'We haven't discussed it any further if that's what you're asking.' Mary risked a glance at her husband in the mirror and saw him looking sceptically at her. 'You can ask him if you don't believe me.'

'It's not that I don't believe you as such, more that it's not like you to let something go when it's been getting under your skin as much as this has.'

'And what would be the point of me pursuing the subject when – as you have told me yourself on more than one occasion – Tom quite obviously does not wish to discuss it?'

'Lady Mary Talbot, are you telling me you actually listened to my advice?' Henry asked, a delighted grin creasing his mouth.

'You needn't sound so surprised. I have been known to heed advice in the past,' Mary said, beginning to feel irritated with him, something that was happening more and more these days.

'Once a flood, maybe, but it's hardly what one would call a regular occurrence,' Henry asserted, looking pleased with himself as Mary slid under the covers. He reached across the bed for her. 'Perhaps we could celebrate the occasion.'

Mary twitched away from him. 'I don't think so, Henry. Not tonight. My head is bothering me.'

'Again?' Henry said, looking displeased. 'It's been bothering you a lot lately. I think perhaps you should make an appointment to see the estimable Dr Clarkson.'

'Perhaps I will,' Mary responded, switching off the lamp beside her and turning her back on him.

'Are you here alone again? Where's Tom?'

Mary looked up from her desk as her father entered the office and scanned the room as if expecting to find his son-in-law hiding behind the coat rack.

'He's at the dealership today.'

'Again? He seems to be spending more and more time there at the moment.'

'Well, he does own it alongside Henry,' Mary said, keeping her voice level.

'But he's also the joint agent for this estate. I hope he's not leaving you to do all the heavy lifting, Mary. I would find that most disappointing,' Robert said, disapprovingly.

'He's not, Papa, I promise. He's doing his fair share of the work, it's just that it's easier for him to do estate business from the dealership. That way he can better juggle the demands on his time,' Mary said swiftly, jumping to Tom's defence, not willing to have her father criticise him unfairly.

'Hmph, I suppose that makes sense in a way. I'm just so used to seeing you two work side by side, it seems odd that Tom never seems to be here anymore.' Robert gave his daughter a thoughtful look. 'That's all it is, is it? Tom trying to manage both businesses at the same time?'

'Of course. What else would it be?' Mary said, attempting to sound surprised by the question.

'You haven't fallen out, the two of you?'

'No, not at all. Why would we have?'

'Hmmm, well, it's just that you and Tom used to practically live in each other's pockets and now you don't,' Robert said, still looking at her consideringly. 'It occurred to me that you might have had words again.'

'No, Papa, we haven't.'

'You've given up interrogating him about his mystery correspondent then?'

Mary's stomach twisted at the mention of the letters that still arrived every morning for Tom. Each day, she watched him take his latest letter and tuck it into his pocket without comment. Each day, another little fissure crackled its painful way through her heart.

'Yes. I've accepted that it's none of my business. It's up to him whether he wants to discuss the matter or not.'

Her father narrowed his eyes at her. 'That's very reasonable of you, Mary. And most unlike you if I may say so. Normally, when you want to know something, you don't let the subject drop. You are quite like your grandmother in that respect.'

'Perhaps I learned my lesson, Papa. I want Tom to be happy and I could see that questioning him about his letters was doing precisely the opposite,' Mary replied, willing her father to drop this topic of conversation.

'Well, good. I'm glad to hear you're not fighting with Tom anymore. It makes the world feel off-kilter when you two are at odds.'

'Was there something you wanted? Or did you just pop in to say hello?' Mary asked, wishing he would leave and let her get back to her work and the ever-present challenge of not thinking about Tom.

'Ah, yes, actually, I do have something I wanted to discuss. I saw Mr Mason earlier and he was telling me about the Malton Fat Stock Show. I was wondering if we ought not enter more animals in the competition.'

For the next twenty blissful minutes, Mary didn't think about Tom, instead discussing with her father the pros and cons of entering too many animals in the stock show and ending up competing too much against themselves. She counted it as a win.