I've taken the opening of this chapter directly from the show and then imagined the rest of the scene from Mary walking off.

'Why were you all in a huddle at breakfast?' Mary asked as she, Tom and Edith strolled beside the river at Brancaster with Rose and Atticus.

Rose beamed, glancing proudly at her new husband. 'Atticus has been offered a job in New York.'

'Oh, congratulations,' Mary said, wondering sourly why everyone was suddenly so keen on going to America.

'Thank you.' Atticus smiled at her then looked over at Tom. 'Aren't you going to America soon?'

Tom cast a quick, tentative glance at Mary, catching the tension in her face as she looked away. 'I am. I'm going to Boston.'

'When?'

'I'm planning to spend Christmas at Downton and leave in the New Year.'

Mary pasted on a smile, aware that Tom was keeping a close, anxious eye on her. 'Well, it hasn't been decided yet.'

Edith chimed in. 'Poor Mary. She hates to be left behind when everyone else is getting on with their lives.'

'It isn't that,' Mary said, her voice as sharp and vicious as a stiletto. 'It's the thought of being left behind with you.'

She turned on her heel and strode off, leaving an awkward silence and an embarrassed, outraged Edith in her wake.

'Mary!' Tom's voice came behind her. She didn't slow down, too angry and upset.

'Mary, stop.' Suddenly he was beside her, his hand on her elbow, pulling her to a halt. 'That was a bit mean, what you said to Edith.'

'She deserved it!' Mary said, venomously, glowering at the distant figure of her sister retreating back the way they'd come, Atticus and Rose fluttering soothingly around her.

'No, she didn't.'

'Yes, she did with her stupid, nasty comments.'

Tom sighed. 'You shouldn't take it out on her. It's not her you're angry with.'

'Yes, I am!'

'No, you're not. You're angry at me for going.'

Mary glared at him, her temper spiking. 'Well, yes, now you mention it, I am. I am angry with you. I'm livid with you. I'm absolutely bloody furious that you're leaving.'

'I know. And I'm sorry.'

Mary felt an immense rage descend on her at his words. 'Sorry? You're sorry? Don't lie, Tom. You're not sorry at all. Not in the least!'

Suddenly, she had to get away from him, had to walk away before all the words bubbling up inside her could escape and she said something she'd regret. She whipped round, storming off up the path away from the castle, away from her wretched sister, away from him.

But Tom wasn't having that. He strode after her, grabbing her by the arm. 'What the hell is that supposed to mean?'

Mary whirled around, eyes blazing, yanking her arm free. Tom took an involuntary step back at the sheer anger radiating off her.

'Sorry? Sorry, my foot! If you were sorry, you wouldn't be going. You'd be staying to help me keep the estate afloat instead of buggering off to America when we need you the most!' she yelled at him. 'If you were sorry, you wouldn't be dragging poor Sybbie halfway across the world, yanking her away from the people who love her best to a place where she knows absolutely nobody! If you were sorry, you wouldn't be leaving me here on my own to do everything by myself! You'd be staying right here! But, no, you want to go and play with cars in bloody, sodding Boston! So, off you go, abandoning the rest of us because you're too selfish to stay!'

Tom gaped at her, then she saw his temper ignite, anger flaring in his eyes.

'I'm selfish? Me? That's rich coming from Lady Mary Crawley!' he shouted back at her. 'You never do anything unless there's something in it for you. You're so high and mighty and entitled, you never even notice the sacrifices other people make for you! You think the world revolves around you and the rest of us are here just to serve your every whim! Well, I've got news for you, milady. You're not the centre of the world. You're certainly not the centre of my world. Sybbie is, and I'm going to Boston for her, not for me. I know you don't like it, but frankly, I don't care. I can't stay here just because I'm a convenient cock for you to come on!'

Mary's jaw dropped, shocked by both the vulgarity and the sentiment of the last bit of his outburst. Fury shot through her.

'Is that really why you think I want you to stay?' she spat, getting right in his face, absolutely livid.

'Isn't it?' Tom snapped, going toe to toe with her, just as furious.

'Yes, yes, it is. Because that's all you are to me, Tom! A warm body for when I get bored! Nothing else!' she snarled. 'But you're easily replaceable, in both the office and the bedroom, so piss off to Boston, why don't you! See if I care!'

They stood glaring at each other, faces inches apart, tension thick between them. Then Tom slammed his mouth onto hers and Mary pushed back, lips, noses and teeth crashing together, hands grabbing and dragging at each other in a frantic push and pull.

'Stop, stop!,' Tom ground out, grabbing her arms and thrusting her away from him.

Mary stilled, panting, her heart beating wildly.

He grabbed her hand and dragged her a few yards to the cluster of trees by the riverbank, pushing through the sweeping fronds of the weeping willow to swing her around and shove her none too gently against the trunk of the tree. And then they were right back at it, attacking each other with hands and lips.

Mary groaned tipping her head back against the tree as he bit down where her neck met her shoulder. She shoved her hips forward, feeling Tom's arousal hard between them and rubbing herself desperately against him. He slid his hand under her skirt, lifting her leg up to curl around his hip as they ground together, rutting frantically against each other.

'Oh, Christ, Mary,' he mumbled, licking into her mouth. Mary bit his lip, tongue duelling with his in a hot, messy kiss, desperate to get him as close to her as possible.

She shoved her hand between them, fumbling with his fly to get at his cock, wrapping her fingers around it, yanking at him. She jerked him fast and hard, her fingers tight and unforgiving, angling her hand to bump against her own sex. Tom leaned against her, propping himself up with one forearm flat on the tree, panting hard next to her ear.

'Mary, stop' he keened, pulling away from her in the nick of time to spill on the ground beside her.

Mary eyed him warily as he turned back to her, his eyes smouldering. He moved forward, dropping to his knees in front of her, bunching her skirt in his hands to push it up towards her waist.

Mary gazed down at him, eyes wide, her brain trying to catch up, as he wrenched her underwear roughly aside, braced his hands on her hips and shoved his face against her to lick and suck and bite at her. She grabbed at the tree, fingers scrabbling at the bark. She arched her back as his tongue found exactly the right spot, biting her lip to stop herself crying out as she rode out her orgasm, bucking against him.

When she stilled, Tom sat back on his heels, replacing her underwear and smoothing her skirt down her thighs, even while her legs still shook. He wiped the back of his hand across his mouth and stared at her shoes, seemingly shocked by what they'd just done.

Mary slithered down the tree trunk to sit like a ragdoll at its base, trying to piece together exactly what had happened in the last few minutes. How they'd gone from raging at each other to ravaging each other in the blink of an eye.

Tom didn't look at her, pulling out his handkerchief to wipe himself clean and make himself respectable again.

'I can't believe we just did that,' Mary whispered, feeling dazed.

'No. I'm sorry,' he said, awkwardly. 'I didn't mean for that to happen. And I didn't mean what I said.'

'Neither did I,' said Mary, quietly.

He glanced across at her, a searching look in his eyes. 'Didn't you?'

Mary gave a rueful shrug. 'Not all of it, no. I was just so angry.'

'At me.'

'At you, at Edith, at the world.'

Tom moved to sit beside her, picking up her hand. Mary tilted her head to rest it on his shoulder, watching him playing absentmindedly with her fingers.

'You've seen first-hand these last few days why I have to go, Mary. You've seen how Lord Sinderby's butler has treated me. Little slights, downright rudeness, blatant disrespect. He's never missed an opportunity to snub me.'

'Stowell is a small, petty man. I have seen how he's treated you and I've been appalled. I asked Baxter to see if Barrow could do something to bring him down a peg or two.'

Tom raised his eyebrows in surprise. 'Did you? There's no need for that. I can handle Stowell's slights. And, anyway, Thomas Barrow has no love for me either.'

'No, but he won't want Stowell looking down his nose at us. It's a question of pride for him and Downton.'

'Well, whatever Thomas does or doesn't do, Stowell's behaviour has just reinforced my belief that I'm doing the right thing taking Sybbie to America. If she'd been here and he'd treated her badly because I'm her father, I really think I would have punched the man in the face.'

Mary nodded, sadly. 'And he would have deserved it. I know you're going to Boston for Sybbie. I know you think you're doing it for the best. I understand all that. It's just… it's hard for me to deal with.'

Tom didn't say anything, just squeezed her fingers.

'I didn't mean what I said about you being replaceable,' she continued, her voice quiet but earnest. 'That wasn't in the least bit true. I was just so shocked and hurt by what you said. I wanted to hurt you back.'

'I'm sorry I said that. It was coarse and vulgar and cruel.'

'But is that what you really think? That I only want you to stay because of our arrangement?'

'No, of course not.'

'Because I don't. I want you to stay because you're one of the most important people in my life, not because you're good in bed. Although I admit that's a bit of a bonus.'

Tom huffed out a small, soft, sheepish laugh.

'I know you're going, Tom. I know that and I will have to accept it eventually, although probably not until you set foot upon that liner. But that doesn't mean I'm not sad about it. And I don't think that will ever change, so that's something you're going to have to accept too.'

Tom lifted her hand to his lips and kissed it.

They sat in silence for a while, listening to the birds sing and the wind sough through the branches of the trees surrounding them.

'We'd better get back,' Tom said eventually. 'The others will be wondering where we've got to.'

Mary let him pull her to her feet, brushing the earth and twigs from her.

'Hmm,' he said, eyeing her critically. 'You might have to change when we get back to the castle. You look a bit rumpled.'

'That's because you rumpled me.'

He grinned at her. 'I did. And I'm not sorry for that.'

Mary put her hand on his arm to stop him moving off. 'Tom? Will you hold me? Just once before we go back. We won't have any time to ourselves at Brancaster and I just… I just need a hug.'

Tom looked at the sadness on her face and pulled her into his arms, wrapping her up safely. Mary closed her eyes and tightened her arms around his waist.