'I had a letter from Maud Bagshaw today,' Cora announced to the table at luncheon the next day.

Mary paused, glancing first at Tom, who had also momentarily stopped slicing his meat although he didn't look up. She transferred her attention to her mother, waiting to hear what was coming next.

'Really? I thought we might have heard the last of Cousin Maud after she won her battle with Mama,' Robert replied, barely looking up from his meal. 'What did she want?'

'She'd like to come to dinner on Friday with Lucy,' Cora elaborated, flashing a look at Tom, who still didn't raise his head. 'Maud says she's beginning the process of introducing Lucy to society as her heir and she thought we might be a good place to start seeing as we know the background and we're family.'

'Oh, right. Well, I suppose she has a point there,' Robert replied, nodding thoughtfully across the table at his wife. 'We'd certainly be a less brutal introduction to our world for Cousin Lucy than some other families. I wouldn't wish someone like Larry Grey or his ilk on her for her debut into society.'

'Cousin Lucy?' Mary said, sharply. 'Is there a reason you're referring to the former maid as "Cousin Lucy"?'

Out of the corner of her eye, she saw Tom looking at her, his mouth tight with disapproval, but she ignored him in favour of pinning her father with an icily enquiring look.

Robert exchanged a glance with his wife. 'Ah, yes, I forgot you weren't acquainted with all the facts.'

'Then perhaps you had better acquaint me with the facts if we are to entertain "Cousin Lucy" on Friday,' Mary said tartly. 'I wouldn't wish to put my foot in it.'

'Lucy is Maud's daughter. That's why she is to inherit Brampton and the rest of Maud's estate,' her father expanded, carefully.

'Her daughter?' Mary frowned at that nugget of information. 'Then why was she – oh. You mean she's illegitimate.'

There was silence around the table and Mary realised she was the only one there who hadn't known about Miss Smith's lineage.

'Well, I suppose that explains why Miss Smith feels she's entitled to her good fortune,' she said, glaring at Tom. 'Although, by rights, as illegitimate issue, she is not entitled to be Maud's heir.'

'I don't think we need to discuss the intricacies of the situation, darling,' her mother said, reprovingly.

'I just think it's rather a rum do that an illegitimate daughter has no hoops to jump through to inherit a great estate like Brampton, whereas when James and Patrick died, Downton was to go to Matthew as a distant cousin instead of coming to me as the legitimate eldest daughter of the current earl,' Mary observed, her bitterness about Miss Smith opening old wounds.

Her father frowned at her. 'Come now, that was because of the entail, you know that. It was and still is unbreakable. Although thankfully we now have George as the heir. Maud is not encumbered by anything of that nature, so she can bequeath her estate as she wishes.'

'Then isn't Miss Smith the lucky one. Indeed, one could say her luck knows no bounds,' Mary bit out, tossing another glare at Tom, heedless of how it might appear to her parents.

Cora laid down her cutlery, giving Mary a warning look. 'I do hope you will be polite and kind to Lucy on Friday, Mary. This is not a situation of her making and we must do what we can to help her enjoy an easy path into our world.'

Mary pasted on a sickly sweet smile. 'Naturally, I will be polite to "Cousin Lucy", Mama. It would be hypocritical of me not to be, would it not, given that she is Maud's Marigold.'

'That's enough, Mary!' her father barked, appalled by her conduct. 'You will behave as befits the daughter of an earl. I will hear no snide comments or unkind words from you on this subject. Your mama is right, we must welcome Cousin Lucy to Downton and do so with good intentions. I will not have you embarrass us with this mean-spirited, dare I say, childish behaviour.'

Mary stared mutinously at her father, clenching her fists angrily on her lap.

'Do you understand me, my girl?' he said, squaring up to her with the full weight of his authority as her father and the Earl of Grantham.

'Yes, Papa,' Mary ground out, holding back her temper with great difficulty.

'Good. Then we will say no more about it,' Robert replied, picking up his knife and fork again. 'Cora, please inform Maud we would be delighted to receive her and Lucy as our guests for dinner on Friday.'

Mary seethed quietly as the four of them resumed their meal in silence.

Mary stalked into the office, Tom following warily behind her, and hurled her handbag at the wall with a frustrated cry.

He braced himself as she whirled on him, pointing an accusatory finger at him. 'You knew!'

He said nothing, just stared steadily back at her.

'You knew she was Maud's daughter, and you didn't tell me!'

'Because it was none of your business.'

'It's like Marigold all over again! You kept that from me too.' Mary flung the accusation at him.

'Because, again, that was none of your business.'

'None of my business? Marigold is my niece!'

'But Lucy's not.'

'No, maybe not, but she is your… your… whatever she is!' Mary snapped.

'It doesn't matter what she is to me, she's nothing to you. I won't discuss her private business with you just as I would never discuss your private business with her,' Tom said, calmly. 'I don't know why you're so upset about finding out why Lucy will inherit Lady Bagshaw's estate. It has no bearing on you whatsoever.'

'Papa humiliated me back there because I didn't know!' Mary cried, still livid about the whole episode.

'No, he didn't. He simply pulled you up when you were behaving badly,' Tom said, not pandering to her.

Mary glared at him, incensed that he was taking that tack.

Tom didn't give an inch. 'Don't give me that look. You know you behaved badly. There was absolutely no need to bring up any of that, especially Marigold's status. You being in a snit was nothing to do with Lucy being Maud's daughter. You're just upset that she'll be here on Friday.'

'Did you know about that? Did you know she was coming?' Mary asked, crossly. 'Because if you did, that's unforgivable. You said you wouldn't spring anything on me.'

'No, I didn't know. I was as surprised as you when Cora said it,' Tom replied, placidly.

'But you're delighted, I suppose! Your… your… fancy piece being invited to dinner!' Mary retorted, anger thrumming through her.

'My fancy piece?' Tom said, raising an eyebrow. 'Is that what she is?'

'Well, what do you want to call her? Your intended? Your paramour? Your lover?' she snarled, getting in his face.

Tom narrowed his eyes at her then took hold of the tops of her arms and gave her a gentle push away from him. 'She's not my lover.'

'She wants to be. She's made that perfectly clear,' Mary scowled. 'For all I know, all those letters you and she write so diligently to each other may be full of all the sordid little details of what you want to do to each other in bed!'

Tom's lips quirked into an amused smile. 'You think we're writing dirty letters to each other?'

'I don't know! That's the whole point!' Mary glared at him. 'Don't you dare laugh at me!'

'I'm not,' Tom protested, trying hard to keep his amusement under control. 'But you've got to admit, it's hardly likely, is it?'

'I don't know, Tom! What else can you possibly write to each other about day in, day out?'

'Everyday things mostly. What's happening in our lives, the news, the weather, that sort of thing.'

Mary stared at him, suddenly feeling marginally better about Miss Smith's letters. 'How very dull of her. I want you to know if I'd been writing to you every day for this long, I would definitely have become a little graphic in my correspondence by now.'

Tom grinned. 'Would you really? Then maybe it's a shame we live under the same roof.'

'Maybe it is. I'm sure you'd find my letters far more entertaining,' Mary said, confidently. 'Although I'd far rather see you every day than only have you present by letter. Even the most explicit letters.'

They stared at each other, the atmosphere between them charged. Tom cleared his throat, breaking eye contact, and walked over to his desk.

'Well, we don't write explicit letters, Lucy and I. They are perfectly respectable.'

Mary gazed at him, feeling hope rise in her chest. 'Maybe she doesn't see you as a lover after all then. Maybe she sees you as a sort of brother type figure.'

Tom stilled, pausing before turning to look at her. 'She doesn't see me as a brother, Mary. Don't you think I would have told you if that were the case?'

Mary felt her hope deflate and the familiar jealousy begin to rear its head again. 'And I suppose I will have to sit and watch her flirt with you all night on Friday then?'

'I don't know, but probably not. Lucy's not the overtly flirtatious type. She's really quite shy. I doubt she'll be all over me like a rash if that's what you're thinking.'

'I'm dreading it, Tom. Seeing you with her,' Mary blurted out. 'I don't know how I'm going to cope with it.'

Tom looked across at her, his face full of concern. 'Please be kind to her, Mary. I know it won't be easy, but please don't take it out on her. None of this is her fault.'

'Are you looking forward to it? Friday?' she asked, curious about what he would say.

'Honestly? No, not really. I think having you both in the same room is going to be difficult. I'll be worried about Lucy, worried about how you're feeling. But I need to focus on her because Friday is going to be important for her. Please don't ruin it all with a scene.'

Mary let out a sigh. 'I won't make a scene, I promise. I can't, can I? I know I have no right to be jealous even though I'm positively seething with jealousy. And besides, Henry will be there.'

'So, it won't be a walk in the park for either of us then,' he said, resignedly.

'No, I suppose not. We'll just have to get through it as best we can.' Mary blew out a sigh and walked over to her desk, bending to retrieve her handbag, peering inside it to see if anything had broken when it hit the wall.

Tom settled down at his desk and began to work. Mary watched him for a minute or so then pulled her compact out of her bag, sighing again when she opened it to find the powder inside cracked and spilling out. She made a mental note to ask Anna to order a new cake of foundation, trying to be thankful the mirror hadn't cracked. The last thing she needed was seven years' bad luck.

Tom was suspiciously quiet all afternoon, head bent at his desk as he wrote and wrote and wrote, taking frequent breaks and often sucking on the end of his pen, a habit Mary found herself avidly watching. She made them a cup of tea around half past three, counting it as penance for her outburst earlier. She saw him cover up whatever he was writing when she came to give him his cup of tea and her heart sank as she realised he must be writing to Miss Smith. (She would be damned if she was going to call her rival Cousin Lucy.)

Not long before five o'clock, Tom finally stopped writing. He folded his letter up and put it in an envelope. Mary watched him seal it up, a dull feeling deep in her soul.

'Shall we head back?' he asked, looking over at her.

'Why not?' she said, listlessly, getting up to retrieve her coat and hat.

As she shrugged her coat on, Tom rounded his desk, a long, slim envelope in his hand. He tilted it towards her.

'Here.'

Mary looked at it in confusion, wondering why he was giving her Miss Smith's letter, and then she saw the letter M on the front of the envelope. She glanced back up at him. 'What's this?'

'Read it when you're alone and then burn it,' he said, enigmatically.

Mary frowned at him, but took the envelope, slipping it into her handbag.

'Promise me you'll burn it,' he said as he pulled his hat into place.

'Why? What is it? Some kind of seditious plot to overthrow the throne?' she joked, wondering what on earth had got into him.

'You'll understand when you've read it,' he answered. 'Now promise me.'

'All right, all right, I promise,' Mary said, suddenly itching to read what was in his letter.

When they got home, Mary excused herself to change her clothes. In the privacy of her bedroom, she delved into her handbag pulling out Tom's mysterious letter. She sat down in the chair by the window and turned it over, carefully opening the flap and pulling out the folded pages.

As she began to read, her eyebrows rose further and further. It appeared Tom – the man who would not tell her he loved her – had spent the better part of the afternoon writing down exactly what he would like to do to her in intimate detail. He'd written her the dirty letter she'd accused him of writing to Miss Smith.

Mary smiled, utterly thrilled by this unexpected turn of events. She settled in to enjoy his letter, knowing deep down that despite promising him she'd burn it after reading it, she couldn't bear to do that. She'd be keeping this particular billet-doux for a long time to come, secreted in an appropriately private place. She had a feeling it would help her through the difficult times ahead in her complicated relationship with Tom. Not least Friday night when Miss Smith came to dinner.