Doyle got an SOS from a close friend at Grove Hill Police Station. It was a personal matter, a family matter. DS George Carrington wanted Doyle to hold his hand (though he didn't put it in so many words!). George's sister, Helen, had recently married some chinless wonder from the Civil Service. The man, Henry, could bore for England. It seemed that only Helen had any regard for him. George and his brother, Philip, couldn't stand the man and couldn't understand what Helen saw in him. Mother, Mrs Frances Carrington, thought it a good idea for the family to get to know Henry a bit better to ease tensions, and had organised a family picnic by the river. Knowing her children's reaction to this scheme, she had softened the blow by agreeing that they could bring a friend or two.

"Why is there never a decent murder when you want one?" George had lamented to Doyle in the pub as the dreaded date came nearer. "Even a suspicious suicide or arson would do."

"How about a full blown riot?" Doyle suggested teasingly.

"Can you arrange one of those?" George had asked desperately!

"I haven't got that much clout, George."

So the day dawned clear and bright. Even the weather was on Mother's side. A small and reluctant crowd gathered in tense and moody silence by the riverbank as Frances tried to appease all sides with forced gaiety.

"Do you think she's overdoing the back-slapping bonhomie?" George whispered to Doyle as Frances's shrill voice carried across the water.

Doyle didn't want to be disloyal to his hostess, so diverted the gloomy conversation. "So, no riots or suspicious deaths then?" he queried.

George smiled at the obvious change in tack. "No, but I do have a puzzle to tickle your brain cells."

Doyle was instantly intrigued, as George hoped he would be. They wandered away from the group with their food as Henry was expounding his theory of statistical analysis. Even his wife looked bored. As her attention clearly wasn't riveted to the speaker, Frances noticed her son and his friend sneaking away from her coterie.

"George, where are you going?"

Doyle couldn't help but smile. At 37 and a senior policeman, George could still be cowed and embarrassed by his mother.

"Spot of business, mother," he said deferentially.

"Won't be too long," Doyle added supportively.

Frances looked hard at them. The word 'betrayer' could almost be heard, but she was too much of a hostess to tackle her guest.

Out of hearing, they settled down at a picnic bench by the bridge with their glasses of wine and plates of very tasty home-made food. Frances had obviously gone to some trouble to make the day work.

"Are we meant to be having a real conversation, or a pretend one?" asked the shrewd Doyle, watching a kingfisher flash across the river's edge and disappearing into a weeping willow.

"The puzzle I've got for you is real enough, if you want to hear it."

"I'm all ears."

"Remember the big smash on the M25 a week or so back?"

"Of course. Four dead and, what, 14 injured?"

"Well, it's now five dead. Of the 16 injured two had to have a leg off. The lorry driver swerved to avoid something - we think it was a dog - running across the motorway. Unfortunately, being foreign, he instinctively swerved the wrong way - into the traffic."

Doyle put on a sympathetic face. It was a tragic accident, but he couldn't see any mystery there, unless the foreign driver (Spanish he seemed to remember) threw up something interesting. He waited.

"There was a family wiped out - parents and their teenage daughter."

"That's right. The papers made a big thing of that."

"Well," George said, looking round conspiratorially, "The autopsy reveals that the girl couldn't be their daughter. Wrong blood group. And, there was no evidence that the mother had ever given birth."

"Virgin?"

George pulled a face. Doyle wasn't taking this as seriously as he'd hoped. "No, but the medical report …"

"All right, don't go into detail. The kid was probably adopted. Or they were running someone else's child somewhere."

"No the girl's meant to be theirs all right. The family, the Alcrofts, lived in Harrow. The neighbours thought the parents were from Birmingham way originally, but have lived in Harrow for years where Ruby was meant to have been born. Since she didn't appear to be biologically theirs, we did a spot of discreet enquiry at the Social Services at Harrow and they say that Ruby certainly wasn't adopted locally. Our mole dug a little further and hacked in to the central database," Doyle raised an eyebrow but said nothing, "and no Alcrofts have ever adopted or fostered. The mole was curious and looked at Rubies. Not a common name but she came across three fostered and three adopted across the UK. None matched our girl."

"You have been thorough."

"Well, we can't do anything too much openly as it's not a police matter. A mystery, but not an illegal mystery as far as we can see so far."

Doyle sat back, relaxed and enjoyed the sunshine. He noticed through half-closed eyes that Henry was still holding centre stage. Even Frances seemed to have been verbally clubbed into a subdued silence. George saw Doyle's keen mind at work as he pondered his puzzle. It had been a while since George had seen the process and it was always a treat.

"I assume that the Alcrofts didn't suddenly appear one day with a child. I mean, the neighbours whom the reporters have interviewed have never said - as far as I remember - that a mysterious fairie child had come amongst them," he said eventually, his eyes closed against the sun.

"True. We can't interview the neighbours about it because that would start tongues wagging and minds whirring."

"Hmm," Doyle murmured softly. George waited. "That would suggest that Mrs Alcroft went through some sort of phantom pregnancy, stuffing ever larger pillows up her jumper. She - and her husband - would have done well to keep that up for nine months. Perhaps she went away for a long period during this 'pregnancy' saying that she needed constant medical supervision - history of miscarriage, danger to the mother, that kind of thing."

George nodded slowly. That made sense.

"Then she came back from this alleged nursing home with a babe in arms," Doyle continued. "Do we have any history of Ruby by the way?"

"Can't find a birth certificate but there is clear evidence that she started at the local infant school and moved normally through the ranks."

"Bright kid?"

"So-so. Nothing brilliant; nothing worrying either. Certainly never came to the notice of the local bobbies, or Social Services either."

"Hmm," Doyle mused again, enjoying the warm sun on his face. So far he only had a vague outline of the child; her bare bones. For her to have flesh and form he'd need to speak to her friends, class mates, teachers, neighbours and so on but, as George had said, this wasn't a police matter (much less a CI5 one), so that was out. He would have to know Ruby theoretically.

After a while he asked, "What was the father's, or the mother's, job?"

"He was a history teacher; she was a clerk at a bank." George didn't know why Doyle was asking but he knew from experience not to query his friend's lines of thought.

"So, during his time at school, Ruby's father could have come across a pregnant school girl - I'm not saying his child - but clearly an embarrassment for the girl's parents and something they'd want hidden."

"Yeah. We dismissed the love child theory, as Ruby didn't have traits from either parent. So what are you thinking?"

"What if the Alcrofts couldn't have kids of their own? They, or he, discover a pregnant student. They approach the child's parents and do a deal that they would take the baby after it was born."

George nodded. "Yeah, that would work."

"Money needn't change hands," Doyle continued. "It suits both sides to keep quiet."

George nodded, but saw that Doyle still had a distant look in his eye.

"Yeah, but why not just go through the usual channels and adopt? I can't readily see an impediment," George persisted.

"Quite," Doyle murmured thoughtfully, then fell silent for a while. "No birth certificate, you say?" he queried eventually.

"No. In fact we found very little paperwork at the house."

"Any relatives?"

"None that we can find. The pair made a will and that may tell us something when it's dug out."

"There is one, then?" Doyle hadn't expected that.

"Yeah. A local solicitor confirms. Unfortunately he's being difficult. Can't read it until and unless we find either a living relative or come up with a death certificate for one."

Doyle smiled secretly.

"What?"

"Just had a nasty thought."

"You think they're international terrorists, or sleepers?"

"You've been reading too much Le Carré. No, what I was thinking of was this. You say that the Alcrofts didn't have kids of their own?" George nodded. "What if that's not for medical reasons?" George looked confused. "What if it's not because one or other was infertile. What if it's because they're not married at all."

"You can have kids out of wedlock now, Ray."

"Yeah, but you can't adopt, I don't think, if you're not married. So why didn't they marry if they were that keen to have a child? Why all the secrecy?"

George could see that Doyle had thought of an answer to that one. His eyes were dancing, and he had that secret smile. His friend had obviously caught onto something George couldn't see yet. George gave it up and shrugged.

"The one big reason I can think of why there might be a lawful impediment to this couple being united in holy matrimony," Doyle explained, "is that rather than man and wife they were brother and sister!"

George gaped and said nothing for a while as the implication sank in. "Phew. I never thought of that. Their blood group was the same. We never looked beyond that. Hadn't got round to looking for a marriage certificate with the Registrar yet either. I'll …"

"I suggest you don't." Doyle leant forward to emphasise his point. "If they are - or were - siblings, what difference can it make now? It sounds as though Ruby enjoyed a stable and happy childhood, so why dig?"

George thought for a while, struggling with his conscience, and then nodded thoughtfully.

"Time to join the party?" Doyle suggested gently, nodding towards an increasingly desperate Frances.

George smiled and got up slowly and reluctantly. He'd have much to report to his boss in the morning.