Disclaimer: Jennifer Worth and Heidi Thomas run the Call the Midwife practice. I'm just a locum.

It wasn't until after something which Jack Smith said that Timothy started to worry. Stretched out on the floor on the parish hall, panting and sweating after a particularly energetic tussle at British Bulldogs, he was smugly satisfied at his win, a historic victory for speed and guile over brute force, when he noticed Jack's pink face peering at him quizzically.

"Is it true your dad's going to marry a nun?"

Timothy grinned. "Yes. Sister Bernadette. Except she's actually called Shelagh." Days on, it was still an almost impossible delight to him. "We beat you in the three-legged race at the fair," he added, unable to resist the temptation to rub that in, fresh from his more recent conquest.

However, Jack was still confused. He had heard the women in his building pecking over the subject with sourly excited voices, until his mother had told them to leave it out and boxed his ears for listening in. "I fort nuns ain't allowed."

"She's not a nun any more. She's," he tried to recall the expression he had heard her use during that strange drive back to Poplar, "'left the Order'."

Jack frowned at this; it was a puzzling thought. "So, she decided she don't want to be a nun no more?"

"Yes. She'll still be a nurse though, when she's properly better."

"And now she's going to marry your dad?"

"Yes."

"Did she stop being a nun because of your dad?"

Now it was Timothy's turn to puzzle. Chasing into winding country lanes in pursuit of a nun-who-wasn't-a-nun was so staggeringly unlike the usual torpor of half term that initially Timothy hadn't considered why they were doing it. He had hung out of the window as they tore through the mist, in tremendous excitement at seeing his butterfly correspondent again, never reflecting on what had caused his father's sudden burst of manic animation, after long months of silence and sadness. He simply assumed it was the usual reason: his father desperately trying to get to somewhere he should have been half an hour before.

Then came a moment when he watched from the car and saw his father and Sister Bernadette together, gazing at each other as if they were the only secure points in a spinning world. His father had clutched his coat around her, cradling her within it like precious fragments of something beyond price, and Timothy slowly began to realise.

In the corner of the edge of memory, he recalled another time when his father had clutched a beloved person within a different coat. It had been himself. He was shivering and soaking, freshly pulled from a river he had tumbled into on holiday. His father shook while he wrapped the coat around him, rubbing his arms and shoulders and berating him in a high, stammering voice Timothy did not recognise. He thought he was angry, until he saw his father's face, its blind terror and boundless love. A stabbing combination of guilt, security and love overwhelmed him as he flung his arms around his father's neck and started to cry. Watching him now was sweet and sad and strange; for a brief moment he wondered whether Sister Bernadette was feeling the same ecstatic pain which he in that second could recall so vividly.

When they had their man-to-man chat over hot chocolate that night, Timothy understood long before his father completed the faltering explanations: he had loved Mummy very much and would never not love her, but now he had started to care for Shelagh (not Sister Bernadette anymore, Timothy) just as much. Incredible though it was, she cared for him too, and he wanted Timothy's permission to court her. A chocolaty beam was smearing across Timothy's face, yet his father was still stumbling over the last apprehensive words; if Timothy didn't mind, he wanted to ask her to marry him. They had visited the jewellery shop together to select a ring; it was Timothy who had devised the plot with the wrapping paper.

Yet Timothy had never considered why she had changed in the first place, why she was "Shelagh, not Sister Bernadette anymore". And as he pondered this, he knew there was something odd about that. If it had been a cocoon turning into a butterfly, he would have investigated at once, demanding answers until he received them or pouring through the encyclopaedia in the sitting room. Weren't these changes – the wrong clothes, not being a nun anymore, a new name – as strange as a butterfly emerging? She had been ill, of course, too ill and far away for Dad to let him visit her, even though he had asked more than once, wheedling and even pointing out that she would have come to visit him if it had been the other way around, something his father did not deny. But did that make things change so much? The jigsaw lay in pieces in front of him: the change of name, an illness, her suggesting they run the race together when his father left, his father writing letters to her, the wrong clothes, Dad sitting in the car watching it rain, the way she soothed and teased him when she bandaged up his arm. It vexed him. He shuffled them in his mind, over and over. One last piece needed to put them in order, one last piece to make it clear. Then he knew it:

"Where are we going then?" Wheels shrieked. No arguments now, no disagreement. He wasn't being left behind or told to stay. Time was too precious.

They jolted out of the driveway. "To collect Sister Bernadette. She's been discharged from the sanatorium." Before the opening mouth could ask its question, "Allowed to leave. It means she's getting better." A clatter of something being dropped by a street vendor at the corner by All Saints. A blur of grey with a cherry red hat shot past on the scooter, too fast to be identified.

"Why didn't you tell me, Dad? I asked to visit loads of times. You were going to leave me behind."

The docks were a kaleidoscope of brown and black, covered in stinking grey. "I only found out fifteen minutes ago. It was her who telephoned just as you were leaving my office." A mechanical scream as his father changed the gear, muttering so quietly Timothy knew he wasn't supposed to have heard, "I won't let you take a bus back alone, not now."

She had telephoned him and they had gone to find her and she wasn't a nun anymore. She had put down her cases when she saw them, but not in surprise. She simply stood and stared and waited while his father approached her in a stumbling half-run. And now she was going to marry Dad. The pieces lay in order and the last twist which brought the last one into its rightful place was Jack's question. Just as he had once instinctively known that his mother was dying weeks before the night when his father finally told him, letting him cry himself to sleep in his arms, somehow, without understanding how, Timothy knew why she had made the decision she had. "I think so."

Jack shrugged. "Alright." He liked Timothy's dad; he didn't really see the attraction of making paper frogs, but Dr. Turner was okay. He laughed at the right bits in the Cubs' performances and everyone agreed he was a very good doctor, who 'knew 'is stuff'. Jack vaguely remembered Dr. Turner telling him years ago that breaking your arm is a stage all men go through and that crying didn't matter, as lots of men he'd treated in the army cried, all the time examining him so gently and deftly that he hadn't noticed the sling being put on. More recently, he had once joined in a cricket match in their street, stripping off his jacket and tie to show the boys how to bowl off-breaks like Jim Laker, playing a couple of elegant strokes across the cobbles and then getting himself out when his score was still comfortably modest. Girls were strange. They giggled a lot and did weird things to their hair. Older girls were even stranger. But if you liked doctors (and Jack supposed that a nurse, even a nurse who used to be a nun, liked doctors), then Timothy's dad seemed a sensible enough choice. He hauled himself upright, leant against the pillar in the middle of the hall and rummaged in his pocket until he found the grubby remains of some polos he had bought with his pocket money.

"Congratulations," he said pompously, rolling each syllable around in his mouth and offering a mint to Timothy by way of a peace offering for his inquisitiveness.

Cheerfully, Timothy took it, grinning as he sat up. "Thanks." In the same spirit of camaraderie, he offered his own modest little boast. "I helped Dad propose! And I'm going to be Best Man."

Jack coughed violently and his face turned even pinker. "You can't! You ain't old enough. You'd look stupid."

That rankled. "I'm only a bit younger than you," he retorted. It would not be totally fair to say Timothy felt that life with the Cubs would be entirely different if he, rather than Jack, were the oldest, but he wondered whether a few extra months (and, even better, a few additional inches in height) would have seen him striding forth as a heroic Robin instead of simpering as Maid Marion. Dad and Sister Bernadette had said that it was because he was a good actor and it was an important role, but he wasn't really convinced.

"It's not that! I ain't old enough! I'd look stupid!" Jack continued. "Best Man has to sort out a knees-up at the pub for all the men and get drunk and dance with bridesmaids and everything. They do loads of stuff."

At that interesting juncture, when Timothy would have been more than grateful for the conversation continuing, the whistle was blown. And that was the point when panic started to set in.

The problem, as Timothy realised later that evening, was that the first person he would normally go to with any question was Dad. Occasionally he would wish he hadn't: the time when he peevishly asked why he had chickenpox, hoping for hot chocolate, a cuddle and another chapter of The Wind in the Willows, and instead was treated to an earnest explanation of how viruses are spread, was probably the all-time low (although he had to admit that he had been given the hot chocolate, two chapters of The Wind in the Willows and several cuddles afterwards). But Dad could be relied upon to give him the answers to questions about pretty much everything, except butterflies, ninety per cent of the time or at least provide him with the right book to find the answer. However he was precisely the one person to whom Timothy couldn't go with this question. If Dad had asked him to be Best Man, then he didn't need to be alarmed by discovering that his son wasn't up to the job. Rather suspiciously, Timothy also wondered if his father had left off the bit about going to the pub, getting drunk and dancing with bridesmaids deliberately for some unknown reason; presumably he did know what a Best Man had to do, given he'd been married before, and that it wasn't just about taking care of a couple of rings.

The second person he would normally have gone to with a question was similarly out of the question: Shelagh-not-Sister-Bernadette-anymore. Timothy wasn't completely sure whether she would have gone to many weddings when she was a nun and of course, she was also a woman, so how could she know what a Best Man did? Additionally there was the fact that while he knew little about weddings, Timothy did know that they were supposed to be 'the bride's day'. However nervous Dad might be at the idea of an incompetent Best Man [a Worst Man perhaps?], it would be even worse for her, especially given she hadn't ever had a wedding before. And beyond all of that, when he remembered how slowly she had let go of his father's fingers before she picked up her bag, got out of the car and walked up the steps to Nonnatus House to talk to Sister Julienne, and how intensely his father had watched her movements until after the door had shut behind her, Timothy wasn't convinced she would be able to keep the predicament secret from Dad.

For a brief moment he wondered whether he could ask one of his teachers at school, but dismissed it immediately: he didn't like being unkind, but he couldn't help thinking that Miss Norris, his new teacher this term, was a little bit silly and not nearly as clever or interesting as Mrs. Fletcher the previous year. However, Mrs. Fletcher was now spending Tuesday afternoons being examined by Nurse Lee or his father at the ante-natal clinic, rather than in front of her blackboard, and even finding out the answer to his question would not make it worthwhile to go through the mortification of speaking to someone by whom he had been taught 'when he was little'. Not for the first time, he thought wistfully about the distant country of grammar school, where he was certain fountains of knowledge would abundantly flow.

He wondered if there was a book which he could borrow on the subject and resolved to search for it at the library on Tuesday afternoon. Mrs. Harrison, their housekeeper, had her day off and unless he was going to Granny Parker's, he always went to the maternity hospital after school, where he was supposed to begin his homework in his father's office and occasionally did. His father would be at the ante-natal clinic, so he wouldn't be missed, and he had a better idea of where the library cards usually were than Dad. Unfortunately for his scheming, when Tuesday afternoon arrived Timothy found himself cheerfully greeted at the school-gates by the smiling face of Shelagh, who had come to surprise him. She was perfectly happy to go to the library and enthusiastically debated the merits of Treasure Island over The Silver Chair as they scanned the shelves, laughing when they were told off for noisiness after he pointed out his favourites a little too energetically. He bathed in her unadulterated attention as they discussed a list he had drawn up of ones she ought to read over cake at a Lyons Tearoom and then meandered to the hospital. He loved his father and knew that his father loved him, but the experience of any adult being so completely absorbed by his thoughts and interests, undistracted by telephones or patients or grief or chores, was something Timothy had experienced so rarely since his mother first became ill that he had no recollection of it at all. However, this blissful afternoon did mean that any chance of wandering into a different section of the library in search of a volume which might not actually exist anyway was non-existent.

Two days passed and his ignorance was still dark and vast. He unsuccessfully tried to devise strategies as he dawdled to school in the morning and as he ran home in the afternoon; even an unexpected third 'thinking session' in the middle of arithmetic had produced nothing. He knew he shouldn't have been thinking about it in school, but it was the third time Miss Norris had tried to explain long division to the class and as he understood before her first attempt (Dad having shown him a couple of months before), he couldn't feel too guilty. As he saw it, when he waited by the lights at East India Docks Road, he had two options, both of which revolved around Cubs the next evening. One was to ask Jack, the other Akela, and neither was very satisfactory. Although Jack was always extremely confident about his views, Timothy wasn't entirely convinced that Jack knew quite as much about things as he always said he did, and although Akela wasn't exactly like most women, and he sometimes forgot she was a woman at all, she was still not a man.

The lights turned and he scanned the street, checking that it was safe, when at last the flash of inspiration, so obvious and there all the time, appeared in front of him walking along the pavement. Instead of crossing, he padded over the salvation, black-uniformed and helmeted, which he had been looking for.

"Excuse me? Constable Noakes? Can I ask you a question, please?"