Mary made it to the bottom of the stairs before collapsing onto the bench there, clutching at her chest, dimly wondering if she was having a heart attack as she struggled to breathe through the pain in her chest.

Barrow came out of the servants' door carrying a bottle of brandy and stopped dead staring at her as she desperately tried to drag in air. Within a couple of seconds, he was kneeling on the floor in front of her. Setting the brandy down, he peeled her white-knuckled hand off the edge of the bench and held it in his own hands.

'Milady, look at me, look at me. Breathe with me, try to breathe with me,' he said, keeping his voice calm and steady as Mary reached out with her other hand and clawed at the front of his livery, her eyes wide with terror.

'I think you're having a panic attack,' he said, putting one hand on her heaving shoulder, trying to ground her with his touch. 'Don't fight it. It will pass, I promise you. Try to breathe in time with me. In… out… in… out… in… out.'

Mary tried to focus on Barrow, fixing her gaze on his mouth as he said the words 'In…out' over and over. Gradually, she began to breathe with him, matching her breaths to his, her racing heart slowing, that unbearable vicelike sensation loosening bit by infinitesimal bit.

'Do you want me to fetch Dr Clarkson, milady?' Barrow asked when he could see the worst was over. Mary shook her head, still not quite able to speak.

The door to the dining room opened and Andrew appeared in the Great Hall, pulling up short in surprise at the sight of the butler on his knees in the hallway holding the hand of Lord Grantham's eldest daughter.

'Er, Mr Barrow, I was just coming to see where you'd got to with the brandy,' he said, awkwardly. 'Is… is everything all right here?'

'The brandy is here beside me, Andy. Take it into the dining room and serve whoever requires it then I want you to pour another measure and bring it discreetly out here,' Barrow told him calmly, keeping his eyes reassuringly on Mary. 'Discreetly, mind.'

'Yes, sir,' Andy said, casting a curious look at Mary as he stooped to pick up the brandy bottle.

'Is it easing a little now, milady?' Barrow asked gently when Andy had gone.

'Yes. Thank you, Barrow, thank you,' Mary croaked, loosening her grip on him.

'Good, that's good. You might feel a bit shaky for a while, so just sit quietly for a few minutes.'

Mary nodded, feeling her breath coming easier now.

After a few minutes, Andy reappeared with a snifter of brandy and passed it to Barrow, who took it and gave it to Mary.

'Just sip it slowly. It'll help you feel better,' he told her. 'Do you want Andy to fetch Mr Talbot for you?'

Mary shook her head, grimacing at the taste of the brandy. 'No, no. I don't want to worry him.'

'As you wish. Andy, go back to your station and not a word of this to anyone. Try to keep them all topped up and in there for a good ten minutes or so.'

'Yes, sir,' Andy said, turning to head back to the dining room.

'How are you feeling, milady?' Barrow asked, still holding her hand.

'Better, thank you,' Mary said, sipping at the brandy again. 'How did you know what it was?'

'I've seen it before in the trenches. We used to carry paper bags back then for men to breathe into. If you can get your breathing under control, it helps the panic to pass.'

'Goodness, I feel a fool now. I hardly think the drawing room of Downton Abbey compares to the trenches of Flanders fields,' Mary said, embarrassed.

Barrow shook his head. 'Panic can affect anyone out of nowhere, milady. Don't feel badly that it happened to you.'

'It's never happened to me before. I thought I might be having a heart attack,' Mary confided in the butler, still feeling the terror of that.

'I've heard men say that. It's the way the panic makes the chest tighten and the heart race. It must have given you a terrible scare.'

Mary nodded. 'It did. Will it… do you think it will happen again?'

Barrow shrugged, something she didn't think she'd ever seen him do before in his stiff livery. 'I've known men who have regular panic attacks and men who have one and then it never bothers them again. Dr Clarkson may be able to tell you more, but I don't think it's all that clear cut.'

'So it could happen again then,' Mary said, dejectedly.

'We've got some paper bags in the pantry. I'll get you some and you can keep them in your handbag, milady. If it happens again, hold your hand around the neck of the bag and breathe deeply into it so the bag goes in and out like a bellows. It'll help calm you.'

'Thank you, Barrow, you've been terribly kind,' Mary said sincerely, genuinely not sure how she would have fared had he not come to her aid.

The butler smiled at her. 'I'd not leave you struggling alone, milady. Besides, Master George would never forgive me if I let anything happen to his mum.'

Mary returned his smile, knowing how Barrow doted on her son and how George adored him back. It was a bond that reminded her of the lifelong mutual affection she shared with their old butler, Carson.

'I don't think you'll want to go back into the drawing room just yet. You might want to ring for Anna first to repair your make-up,' Barrow said, diplomatically.

'Is that your way of telling me I have mascara all over my face?' Mary said, raising an eyebrow.

'Well, you do look a little like Miss Caroline's panda bear, if you don't mind me saying so, milady,' Barrow returned, his lips twitching slightly.

Mary smiled. 'I think you've earned the right to tease me a little tonight, Barrow. I won't be returning to the drawing room. I shall retire for the evening. And I'd better let you get back to your duties before your absence is noted. Would you be so good as to tell Mama and Papa that I have a headache and have gone to lie down if they enquire about my whereabouts? And may I ask that you don't say anything to anyone about this little episode?'

'Certainly, milady,' the butler said, getting to his feet. 'You can rely on my discretion. And I'll make sure Andy doesn't say anything about it either. Would you like me to send Anna up to you?'

'Yes, please, Barrow. I am in your debt for your kindness to me tonight. I am truly grateful you were here,' Mary said, reaching a hand out to squeeze the butler's arm.

'It was nothing, milady. I'm glad you're feeling better,' Barrow said, inclining his head and backing away to return to his duties.

Mary sighed and took another fortifying sip of brandy before rising to her feet, deciding it was time to move before anyone found her sitting there at the foot of the stairs in such a forlorn state.

She turned to make her way upstairs, her legs shaking slightly. She made it to the first turn before she heard Tom's voice calling her. She kept going, keeping her head down, only to hear him bounding up the stairs after her. He caught her hand, forcing her to stop.

'Mary? Are you all right?' he asked, his voice full of concern. 'Is there something wrong?

She tried to turn her head away from him, so he couldn't see her face. 'No, it's nothing. Go back downstairs.'

Tom lifted his other hand to catch her face and turn it towards him. His eyes widened at the sight of her mascara-streaked face. 'What on earth's happened? You've been crying and I know you, Mary Crawley, you don't cry over nothing. What is it? What's wrong?' he said, squeezing her hand.

Mary looked at him, a slew of emotions rushing through her exhausted body. 'Everything,' she whispered.

'What's happened?' he said again, worriedly.

'Lucy… she…'

'She what?' Tom asked, anxiously scouring her face. 'Has she said something to upset you?'

'She was asking about Sybil and… and Matthew… and it… it was just too much.'

Tom frowned, puzzled. 'Why was she asking about Sybil and Matthew?'

'She knows so much about us. You've told her so much,' Mary whispered, trying not to cry, terrified the panic might return.

'I… no, I told her some things, but she had no right to upset you by asking about Sybil or Matthew. Why was she asking about them?'

'Because she's in love with you, Tom! She wants to know about the first Mrs Branson because she wants to be the second Mrs Branson! And who better to ask than your first wife's sister?' Mary cried, struggling to keep her voice down. 'And then she tells me how she knows I helped you through your grief and that led to talking about Matthew and you helping me and suddenly she's talking about all the tragedies we've had to live through.'

'Oh, Mary, I'm sorry. I'm so sorry. I didn't expect her to ask you about Sybil – or Matthew, come to that,' Tom said, stroking his thumb over the back of her hand, his eyes full of apologies. 'I'll speak to her about it. It won't happen again.'

'No, no, don't. It's not even that that's upset me really. I understand why she wants to know about Sybil, I do. She's wondering if she can measure up to her. It's not her fault, really. I was already upset, and her questions just tipped me over the edge.'

'I thought something was wrong at dinner. You were so quiet, so not like your usual self. Why were you upset? Has something else happened?'

Mary nodded miserably.

'What? Is it Henry?'

'No.'

'What then?'

'Granny knows,' Mary said, flashing a guilty glance at him.

Tom frowned. 'Knows what?'

'About us.'

Tom looked at her, unsure what she was saying. 'What do you mean she knows? What does she know?'

'She knows… how we feel. About each other,' Mary said, glancing quickly down the stairs to make sure no-one else was around.

Tom stared at her, bewildered. 'But how does she know? Did you tell her?'

'You went to her, didn't you? After I told you her secret, you went to her and offered to help.'

Tom looked uncomfortable. 'Yes, I did. Did I do wrong?'

Mary reached up a hand to stroke his cheek, her heart full of gratitude. 'You darling man. I love you for that. But she knew you did that for me, so I wouldn't be the only one she was leaning on. And she knew how upset I'd been about your letters. She put two and two together.'

Tom bit his lip, putting two and two together himself. 'So, she knows. And I suppose she doesn't approve, does she?'

Mary shook her head, tears threatening again. 'No, but I don't think it's about disapproval. She wasn't judging us. If anything, I got the feeling she understands, but she's worried about what it could do to the family if we act on it. The scandal of it all. How it could affect Papa and Edith and everyone else.'

'You mean if you divorce Henry?' Tom asked quietly, his heart sinking. 'I know it's not exactly an everyday thing, but it's not like there haven't been divorces among your set before.'

'No, it's not even that, although that's a factor, I suppose. She didn't mention Henry except to caution me to remember I have a husband. It's more about how the rest of the world sees us. You're my brother-in-law. We'd be juicy fodder for the gossips. They'd tear us to bits.'

'I don't care about gossip! I care about you!' Tom said in a fierce whisper, tightening his grip on her hand.

'I know. And I told her you were more important to me than my reputation, but she pointed out that it's not just my reputation at stake. What we do reflects on the whole family.'

'No,' Tom said, shaking his head urgently. 'That's not true.'

'It is, my darling, it is,' Mary whispered, a tear spilling from her lashes and rolling down her cheek. 'I wish it wasn't, but it is.'

Tom lifted his hand, swiping his thumb over her cheek and brushing the tear away. 'No, we're not responsible for other people. Whatever we do, that's about us and only us.'

'No, it's not. My behaviour, it reflects on Mama and Papa. It affects Edith and Bertie. It wouldn't just be our names we'd be dragging through the mud.'

Tom stared at her. 'What are you saying? That it's over before we even had a chance?'

Several more tears trickled down Mary's cheeks as she returned his gaze. 'I don't… I don't see how we can take that next step without ruining the people we both care about. Lord knows, Edith would never forgive us if we destroy Bertie's chances of remaining in favour with the King. You know he's pushing Bertie to show the Prince of Wales how to be the perfect family man. All that will go up in smoke if they discover his sister-in-law is in love with her other brother-in-law.'

Tom shook his head. 'The Prince of Wales won't give a tinker's cuss. You and I both know he's no stranger to affairs.'

'But it's not about him, is it? It's about Bertie and his prospects. And Edith's. And I know you love Edith, even if I think you're a poor misguided fool for doing so. And then there's Papa and Mama. And I don't want Granny's last days to be mired in scandal.'

'So, that's it, is it? There's no hope for us?' Tom asked softly, his voice catching. 'It's back to longing looks in the office and small touches that drive me insane with want?'

'Please don't stop those touches,' Mary begged him, a chasm of despair yawning wide inside her. 'They drive me insane too, but I don't want them to stop. Especially not if that's all we'll have.'

Tom pulled her closer, his arms around her waist, his forehead resting against hers. 'I couldn't stop touching you if my life depended on it.'

'Good, because I can't stop touching you either,' Mary breathed, more tears slipping down her face. She rested her hands on the front of his dinner jacket. 'Lucy. I… I like her. I didn't think I would, but I do. It's annoying. Like Lavinia all over again.'

'I thought you might like her. Well, if circumstances had been different, I thought you might like her. Just not when you saw her as a rival.'

'She wants to be friends with me. She wants me to help her learn how to manage an estate.'

'Yes, I told her ages ago you'd be a good mentor for her. Before you started acting all crazy about her letters, obviously.'

'I know. She told me.' Mary closed her eyes, steeling herself to say what she felt she had to. 'She'll be good for you. I can see why everyone has been telling me you'd be a perfect match. You will be.'

Tom pulled his head back, gazing at her carefully. 'You sound like…'

'Like what?'

'Like you're giving me to her.'

Mary bit her lip, feeling more tears fall. 'I think I am. I think I have to.'

'No,' he said, shakily. 'No.'

'It's not fair of me to keep you… keep you waiting in limbo,' she whispered, definitely feeling her heart break this time.

'No,' he said again, his face full of anguish.

'She loves you, Tom. She'll love you forever if you let her.'

'But I love you!' he whispered desperately, breaking his cardinal rule.

'And I love you,' Mary whispered back, shattering inside at hearing him finally say it. 'But we can't… we can't…'

Tom folded her in his arms, hugging her tight as she cried, blinking back his own tears. Mary clung on, wrapping her arms around his waist.

'Mary?' Henry's shocked voice came from the bottom of the stairs. 'What the bloody hell is going on here?'

Mary pulled her face out of Tom's neck, taking a deep, shuddery breath to try and stop the tears and bottle up her emotions.

Henry ran up the stairs, coming to a halt beside Tom and Mary as they separated. He looked at them uncertainly, his anger at finding them in an inappropriately close embrace wavering as he saw the state of Mary's face. 'What's going on? What's happened?'

Mary sniffed, wiping her fingers under her eyes, seeing them come away smudged with black. 'I was upset. Tom was just comforting me.'

'Upset about what?' Henry asked, looking suspiciously at Tom.

'Miss Smith asked me some questions about Sybil and Matthew and, for some reason, it was all too much for me tonight. It brought back a lot of painful memories.'

'Oh. Are you all right now?' Henry said, obviously thrown by her words.

'No, not really. I'm going to go to bed.'

'Do you want me to come with you?'

'No, no, go back downstairs and enjoy the rest of the evening, both of you, please,' Mary said, quickly. 'And if Miss Smith asks after me, simply tell her I have a headache. I don't want her to feel bad. I know she didn't mean to upset me.'

'All right, if that's what you want' Henry said.

'Will you be all right?' Tom asked softly, catching her hand even as Henry narrowed his eyes at the gesture.

Mary squeezed his hand. 'I will. I probably just need a good night's sleep. I'll be right as rain in the morning, I'm sure.'

'If you're sure. Sleep tight, Mary,' Tom said, leaning in to kiss her cheek despite Henry's look of annoyance.

'I'll be up shortly, darling, as soon as our guests leave,' Henry said, kissing her brow rather than her make-up streaked cheek.

'Take your time, Henry. I really do have a headache, so I think I'll probably be asleep as soon as my head hits the pillow anyway,' Mary said, praying that would be the case. 'Now, go, both of you, before people begin to wonder where we've all disappeared to.'

She watched as the two men turned and went downstairs, Henry still a little stiff with Tom. As they reached the bottom flight of stairs, Tom looked back up at her, his head turned away from Henry.

'I love you,' he mouthed, desolation on his face.

Mary's heart cracked one more time.