Two evenings later, in an affluent dining room in Hampstead, a middle-aged couple sipped coffee. Dry humour lingered around the man's mouth while his wife read the intriguing letter. She was elegantly dressed, her auburn hair beautifully coiffured, but her eyes were filled with a teenager's inquisitive delight and she seemed unconscious of pencil dust dirtying her hands. In the distance, they faintly registered a piano halted by the telephone's cry, but did not move.

"Is this possible?"

He shrugged. "It seems so. I hope so."

"But, has he – "

"No, not yet. It explains why he kept trying to call me last week though."

"He looked so ill the last time we saw him. He looked dreadful, almost as bad as - "

"After Elizabeth. I know." He remembered as keenly as she did: that terrible, terrible brittleness. "What was it he said? His colleague had TB or something?" He leant over and indicated a line in the opening paragraph. "But, darling, if that little detail means what I think it does, it can't have been easy."

She shook her head. "No. Poor poppet. He'll have been in misery." Then she started to twinkle. "Our proper, careful Patrick. Are you sure he never mentioned this to you? Hinted at it?"

"Of course not, Louisa. We couldn't possibly talk about such things!"

"What then? Only of politics and the efficacy of chlorambucil and who'll open the batting for Middlesex next summer? You silly men. Thank God I'm a woman, that's what I say," she added, more coquettish than critical.

He caught her tone instantly and lowered his spectacles on his nose, one eyebrow raised; their chamber piece was long played and by experts. "As I am, my dear, as I am."

Almost twenty years of marriage had not dulled their flirting with each other; the appearance of their teenage daughter, however, was a more potent disincentive.

"Dad. Telephone. It's Uncle Patrick."

With flurries of chuckling they departed the room, his cry of 'Bring the letter' causing a fresh peal of merriment. The young messenger, more grave than her parents, rolled her eyes in mute appeal to an invisible audience, muttering "Why are you two so embarrassing?" before returning to her sonata, while the couple hurried to the telephone, he sitting on a chair beside it, she hovering beside him.

"Hello? Patrick?"

"Hello, David."

"Patrick! Lovely finally to catch up with you. Sorry about last week. How are the two of you?"

"We're well, both of us."

"Good, good. You sound very well. Any particular reason?"

The response was a non-committal noise. "And you?"

"Fine, fine. Pootling along, nothing changes."

"And Louisa? And the children?"

"Same as ever. Louisa keeps me up to date with all the things I'm doing wrong." There was a faint thud and a high pitched squeal, as Louisa boxed her husband's ears, then chortled when he stuck his tongue out at her. "Katherine's life is impossible because nobody understands the trauma of being fifteen, Alex lives to play rugby, apart from when he's got broken bones or concussion from actually playing rugby, and Oliver's decided that school is pointless. I'm terribly pleased that you rang, actually. We need to speak to you about Christmas."

"Christmas?"

He smirked mischievously. "Louisa and I were hoping you and Timothy would spend Christmas with us. I know last year you wanted to be by yourselves and obviously we understood. But this year we can't bear the idea of the two of you, just by yourselves, rattling around in your house. Come for dinner or the day or stay over, it's no difficulty. The boys are more than happy to accommodate Timothy and we've easily got room for one person in the guest room."

There was a short and uncomfortable pause. "Thank you. That's very kind of you, David, but – "

"Do at least think it over," he interrupted. "Genuinely, it's no problem. We'd love to have you and squeezing in two people is hardly an imposition. Besides, Patrick, we know how appalling your cooking is. I prefer it if my godson didn't starve over Christmas. I have this terrible fear of you with nothing better than marmite and toast to go with your presents, unless you're hoping one of the nuns will feed you scraps from their table." He had further embroidery for this tale, how Patrick could change his mind up until the Saturday before Christmas, say, but by now Louisa was close to hysterics and he was in danger of doubling over with mirth and had to stop.

The discomfort had increased. "I – don't think so. There's – something happening at Christmas."

"Really?" said David, briefly covering the handset to laugh. "How intriguing."

"Well, yes." David Watson relented. Much as he enjoyed teasing his friend, his affection was greater and his curiosity rampant after the letter he had received that morning:

Dear Uncle David,

I hope you are well. I am writing because Dad is getting married again. She used to be Sister Bernadette but now she is called Shelagh. (Shelagh is a Scottish spelling. She is Scottish.) She is a nurse, but also great fun and we won the three-legged race at the fair together last summer. Dad is really happy at the moment, which is good, but he keeps whistling which is quite annoying.

I proposed to her for Dad and he asked me to be Best Man. Constable Noakes (Akela's husband) says that Best Man organises a party for the groom before the wedding called a stag do and does a speech at the reception [carefully re-written over a smudgy spot] with funny stories in it. I hope you can help me with this and give me advice. I think Dad would like to go to a play for his stag do. Is this is a good idea and can you come? I can't think of any other friends Dad has except Uncle Kenneth and he lives too far away. Also how do I get tickets? Also please can you and Auntie Louisa tell me some stories about Dad at university for the speech. I don't know any because I wasn't there. I am writing to Uncle Kenneth too. Finally do you know anything about what Dad did in the army in the war? I know he was in a place starting with 'D' in France, but don't know anything else. The wedding is the Saturday before Christmas, so please write back soon.

This is secret from Dad so please send your reply to Constable Noakes. He is helping me and will give me your letter. His name and address are: Constable Peter Noakes, Nonnatus House, Leyland Street, Poplar, E14. He and Akela live there at the moment as their house is being pulled down and she is a nurse.

During half-term Dad and I finished the model Spitfire you gave me for my birthday. It is really great, so thank you very much. I am saving up my pocket money to buy a Lancaster bomber kit to go with it. During half term I earned some extra money for cleaning test tubes, petri dishes and pipetes [as spelt by Timothy], which was very boring.

I hope Auntie Louisa, Katherine, Alex and Oliver are well.

From Timothy

"I'm sorry Patrick, go on. Is there something particular you wanted to speak about?"

"Yes, two things. One just came up today."

"Go on."

"I have a patient I need to refer." The change was instant. No humour, only attention and empathy, as Patrick recounted the everyday and ordinary tragedy: an impoverished family, the child who failed to thrive, familiar symptoms wasting a weak body.

A pen and pad materialised in David's hands, brought by Louisa. "Leukaemia?"

"I suspect so, yes."

"It sounds only too likely. What's the child's name?"

"Gracie Higgins. It will be terrible for them, David. They had a stillborn child last year and Mrs. Higgins has never stopped mourning. I haven't told them my full suspicions yet."

David winced. This second heartbreak was impossibly cruel. "What do they know?"

"That it's very serious and will involve complex treatment. If it is leukaemia, they'll need to be told by degrees. I'm happy to do it, of course, but it must be sensitively handled. I'm very sorry to ask, David, I know how busy you are, but could you personally monitor this?"

"Yes, of course. Refer her to me tomorrow."

"They are desperately poor. A bus to Great Ormond Street will stretch them."

"I'll arrange for them to be collected. We have some resources, not many. Perhaps we can help Mrs. Higgins stay with her daughter. What's the address?"

He quickly gave it. "Thank you."

"Not at all." It was impossible to return to teasing with the bald facts of a family's agony in front of him. Sobered, he spoke again. "There was something else, Patrick."

"Yes. Some news and a favour."

"Whatever it is, consider it done. Good news, I hope." He heard the throat being cleared.

"Yes. Very good, although something of a surprise. I'm getting married again, David."

Both men had imagined this moment. Patrick had feared being too abrupt, David had imagined stumbling, pause-filled explanations. But it came instead with astonished, quiet pride.

David faltered. He was remembering the terrible conversation they had the night that Elizabeth's diagnosis was confirmed. There had been the same quietness in the voice, but then racked with anguish. "Patrick, that's wonderful. Truly wonderful."

"Thank you."

His original intention had been to ask whether she was nurse or nun, those being the only women Patrick met, observing that it must be a nurse, given nuns were out of the question, or asking how the lady felt about bigamy, given Patrick was married to his practice. Haunted by the sudden memory, his wit seemed feeble and facetious now. There was time for humour in the years to come. "We had no idea you were courting someone. Who is she?"

"We weren't really courting. It's somewhat complicated, David." His explanation was unadorned: the long held friendship which had grown into affection, how his affection had evolved so slowly, like the pull of the tide, that he had not realised until it was too late to turn back. He did not mention the kiss upon her hand at the fete. In his kinder moments he still considered it ill-judged clumsiness, in his harsher ones, as unforgivably selfish and taking advantage of her. But through his simplicity, the confusion and pain which had underpinned that moment and so many others was clear. There was no interruption, for David Watson was biting his lip and when he glanced at his wife, whose head lay on his shoulder listening as intently, she was crying.

"It came to a head towards the end of the summer because she became ill." Even now it was distressing for Patrick to discuss, but his inflections told David the truth.

"The colleague who had TB. It was her?" There was a brief affirmation. "My God, Patrick. Again?" He regretted the words as soon as they were said, but their honesty and the clarity of his insight were an extraordinary comfort to Patrick.

"Well, quite. I wrote to her when she was in the sanatorium. It changes your perspective. Our perspective. Shelagh calls it her 'time in the wilderness'. And while she was there, she decided to leave the Order and," even down the telephone line, they heard him start to smile, "we're engaged."

David started to express his congratulations, in words as unembellished and heartfelt as Patrick's. However, a hand was waving in front of his face. Louisa, flicking the tears away from her eyes, was mouthing 'Give me the phone'.

"'It's Patrick, darling'," he began in an exaggerated stage whisper. "Patrick, Louisa's just walked in and wants a word. 'Wonderful news. Patrick's engaged.'"

"Patrick! How lovely! Tell me all about it." The note of surprise was dismally feigned.

While he had found the call emotional, it did not mean that Patrick had lost leave of his senses. "Hello, Louisa. Given you've probably been listening to most of the conversation, do I need to?" She laughed heartily. "You always were an appalling liar."

"Spoilsport!" But the tension eased and an old, familiar silliness re-emerged, back from when she was Miss Twynam and he was Mr. Turner and Professor Davies was berating them for talking too much instead of dissecting their cadaver. "Well, go on, poppet. What's she like?"

Now he could smile. "Her name's Shelagh, she's one of the nurses, she's Scottish."

"Know that already. Like you said, I've been listening for ten minutes. What else? How old is she?"

It was typical, thought Patrick acidly, that Louisa would be drawn, missile-like, to the greatest vulnerability. "A lot younger than me."

"How much? What age is she?"

"Thirty-two in the spring."

Louisa blew an unceremonious raspberry. "That's nothing. That's, what, about fifteen years? It's only marginally more than Ken and Alice."

"That's not nothing, Louisa. I do worry," he began.

"If it wasn't that, you'd worry about something else. You think you're being a dirty old man, don't you?" She continued without waiting for him to admit she was correct. "Well, don't. You're hardly a geriatric, Patrick, and she's not a child. If you want to marry her, I imagine she's fairly intelligent."

"Yes. She's extremely intelligent."

"Right, so she's a grown up, intelligent woman. Do her the credit of accepting she knows her own mind." There was no response and she wondered if she had gone too far. "Patrick? Are you there? Have I offended you?"

"Yes, still here. Not offended, just a touch of déjà vu." he said. "Shelagh said something very similar to me once herself."

Louisa laughed. "Good. I like her already. Now, something more interesting."

"She's an excellent practitioner, could have been a doctor herself if she'd wanted."

There was a sigh. "That's useful if I were going to employ her, but that's not really what we're aiming for. She's going to be your wife, I'd like to be her friend. So, something real please."

For the first time since the call began, he fully relaxed. He remembered how he described them to Shelagh while driving to her lodgings; David, honourable and talented and self-deprecating, as only a man called 'Dr. Watson' could be, his former flatmate, his closest friend; and then Louisa: "Like a hornet. The exasperating little sister I never really wanted anyway. She'll interrogate me into submission about you and then it'll be your turn." Shelagh had chuckled, gravely observing it was better to be forewarned. Now this scene was playing exactly as he had anticipated.

But how could he describe Shelagh truly, even to those who understood him best? He hardly understood himself how she stimulated, stirred and mended him. A tensile mind, robust professionalism, as fine a nurse as he knew; yet this was to make an automaton of her, losing the empathy so integral to her ability. Her beauty was fresh and unflaunting, but he did not know whether it was from her flesh or the loving integrity shining through it. Her gift for love and goodness seemed boundless, almost making him believe in God once more, yet she was no ethereal being. He knew now of her burgeoning passion, her human needs as well as her humanity, and he had always loved the effervescent bubbles of her humour. He remembered one joke she made about why Conchita Warren might be more exciting about than a pair of enemy binoculars, a more ribald version of one she made at the nurses' lunch table. From any other person it could have been dubious, even lewd, yet from her was delightful and naughty and utterly beguiling. She entranced him as a mystery, but he knew and was known by her.

"Come on. I'm still waiting. What's she like?"

"A miracle."

His attempts to encapsulate her seemed woefully inadequate to him; he only knew that he was blessed. But away to the north-west, on the other side of London, Louisa Watson's smile was radiant, while her husband's eyes were filling with tears. Both were struggling when he finished, but Louisa recovered first. "And what does Timothy think?"

"He adores her. She's wonderful with him."

"Good," she said, composure almost regained. "I want to meet her."

His groan was comically exaggerated. "Must you?" But he gave her the number and when he spoke he was serious once more. "Don't frighten her off, Louisa, please. May I have David again, please? I have a favour to ask of him."

"I'll play nice, Patrick, I promise. It's lovely, truly." She wanted to say more, but was too full of feeling to. "Here he is."

"What's the favour?"

"It's about Best Man. We've asked Timothy. It's as much about him as it is about us and we both want him to be part of the ceremony."

"That makes perfect sense. It's an excellent decision. What do you need me to do?"

"Would you mind doing the formalities at the reception? I've not mentioned that side of it to Timothy, he's far too young and it's not the important thing. It will be fairly low key, but I suppose we must have speeches of some sort. I'm sorry, I realise I'm asking you to do the worst part."

David could not keep his face straight, but somehow kept it from his voice. "Not at all. I promise that the formalities will run beautifully and you'll have a Best Man's speech to be proud of."

"Thank you, David. For everything."

"Of course." His voice tightened. "I really am very, very pleased, Patrick. We both are."

"I know."

"Be in touch."

"I will. Love to the boys. It was lovely talking to Katherine earlier."

"The same to Timothy. Tell him I'll be in touch soon. Goodnight."

"Goodnight."

As the receiver was put down they turned to each other, in tearful elation, unsure whether to cry or cry out in joy as they learnt of the cure which they had feared was impossible and discovered the great act of healing which had now taken place.