I was the first of the mothers outside Grindle's, the nursery school in Rawlinson Road. My two hours of smoking with Bérangère had me craving for another cigarette but I didn't like to smoke outside the school so to distract myself I thought about my mother.

My mother, Sally Gilmartin, nee Fairchild. No, my mother, Eva Delectorskaya, half Russian, half English, a refugee from the 1917 Revolution. I felt the incredulous laugh clog my throat and I was aware I was shaking my head to and fro. I stopped myself, thinking: be serious, be sensible. My mother's sudden revelatory detonation had rocked me so powerfully that I had deliberately treated it as a fiction at first, reluctantly letting the dawning truth arrive, filling me slowly, gradually. It was too much to take on board in one go: never had the word 'bombshell' seemed more apt. I felt like a house shaken by some nearby explosion: tiles had fallen, there was a thick cloud of dust, windows had blown in. The house was still standing but it was fragile now, crazy, the structure askew and less solid. I had thought, almost wanting to believe, that this was the beginning of some complex type of delusion or dementia in her – but I realised that, for my part, this was a form of perverse wishful thinking. The other side of my brain was saying: No: face it, everything you thought you knew about your mother was a cleverly constructed fantasy. I felt suddenly alone, in the dark, lost: what does one do in a situation like this?

I tracked over what I knew of my mother's history. She had been born in Bristol, so the story went, where her father was a timber merchant, a timber merchant who had gone to work in Japan in the 1920s, where she had been schooled by a governess. And then back to England, working as a secretary before her parents' deaths prior to the war. I remembered talk of a much-loved brother, Alisdair, who had been killed at Tobruk in 1942… Then marriage to my father, Sean Gilmartin, during the war, in Dublin. In the late 1940s they moved back to England and they settled in Banbury, Oxfordshire, where Sean was soon established in a successful practice as a solicitor. Birth of daughter Ruth occurred in 1949. So much, so relatively ordinary and middle-class – only the Japanese years adding a touch of the otherworldly and exotic. I could even remember an old photo of Alisdair, uncle Alisdair, propped on a table in the living room for a while. And talk too from time to time of émigré cousins and relatives in South Africa and New Zealand. We never saw them; they sent the odd Xmas card. The swarming Gilmartins (my father had two brothers and two sisters – there were a dozen cousins) gave us more than enough family to cope with. Absolutely nothing to take exception to; a family history like hundreds of others, only the war and its consequences being the great schism in lives of otherwise utter normality. Sally Gilmartin was as solid as this gatepost, I thought, resting my hand on its warm sandstone, realising at the same time how little we actually, really, know of our parents' biographies, how vague and undefined they are, like saints' lives almost – all legend and anecdote – unless we take the trouble to dig deeper. And now this new story, changing everything. I felt a kind of sickness in my throat about the unknown revelations that I was sure would have to come – as if what I knew now were not destabilising and disturbing enough. Something about my mother's tone informed me that she was going to tell me everything, every little personal detail, every hidden intimacy. Perhaps because I had never known Eva Delectorskaya, Eva Delectorskaya was now determined that I should learn absolutely everything about her.

Other mothers were now gathering, I saw. I leant back against the gatepost and rubbed my shoulders against it. Eva Delectorskaya, my mother… What was I to believe?

'Fiver for them,' Veronica Briggstock whispered in my ear and brought me out of my reverie. I turned and kissed her, for some reason – we normally never embraced as we saw each other almost every day. Veronica – never Vron, never Nic – was a nurse at the John Radcliffe Hospital, divorced from her husband, Ian, a lab technician in the university chemistry department. She had a daughter Avril, who was Jochen's best friend.

We stood together, talking about our respective days. I told her about Bérangère and her amazing coat as we waited for our children to emerge from the school. The single mothers at Grindle's seemed unconsciously – or consciously, perhaps – to gravitate towards each other, being perfectly friendly towards the divorced mothers and the still-married mothers, of course, and the occasional sheepish dad, but somehow preferring their own company. They could share their own particular problems, without need for further explanation, and there was, I thought, no need for pretence about our single state – we all had stories we could tell.

As if to illustrate this, Veronica was moaning profanely about Ian and his new girlfriend and the new problems that were mounting as he tried to duck out of his appointed weekends with Avril. She stopped talking as the kids began to come out of the school and I felt immediately the strange illogical worry that always rose up in me as I searched for Jochen amongst the familiar faces, some atavistic motherly anxiety, I supposed: the cave-woman searching for her brood. Then I saw him – saw his stern, sharp features, his eyes searching for me, also – and the moment's angst receded as quickly as it had arrived. I wondered what we would have for supper tonight and what we would watch on TV. Everything was normal again.

We – the four of us – sauntered back up the Banbury Road towards our homes. It was late afternoon and the heat seemed to possess extra gravity at this hour, as if it were physically pressing down on you. Veronica said she hadn't been this hot since she'd been on a holiday in Tunisia. Ahead of us Avril and Jochen walked, hand in hand, talking intensely to each other.

'What've they got to talk about?' Veronica asked. 'They haven't lived enough.'

'It's as if they've just discovered language, or something,' I said. 'You know: it's like when a kid learns to skip – they skip for months.'

'Yeah, well, they can certainly talk…' She smiled. 'Wish I'd had a little boy. Big strong man to look after me.'

'Want to swap?' I said, for some stupid, unthinking reason, and immediately felt guilty, as if I'd betrayed Jochen in some way. He wouldn't have understood the joke. He would have given me his look – dark, hurt, cross.

We'd reached our junction. Here, Jochen and I turned left on to Moreton Road, heading for the dentist's while Veronica and Avril would continue on to Summertown, where they lived in a flat above an Italian restaurant called La Dolce Vita – she liked the daily ironic reminder, Veronica said, its persistent empty promise. As we stood there making vague plans for a punting picnic that weekend I suddenly told her about my mother, Sally/Eva. I felt I had to share this with at least one person before I talked to my mother: that the act of retelling it would make the new facts in my life more real for me – easier to confront. And easier to confront my mother too. It wouldn't be kept a secret between us because Veronica was party to it as well – I needed one extra-familial buttress to hold me steady.

'My God,' Veronica said. 'Russian?'

'Her real name is Eva Delectorskaya, she says.'

'Is she all right? Is she forgetting things? Names? Dates?'

'No, she's as sharp as a knife.'

'Does she go off on errands then comes back because she can't remember why she went out?'

'No,' I said, 'I think I have to accept it's all true,' and explained further. 'But there's something else going on, almost a kind of mania. She thinks she's being watched. Or else it's paranoia… She's always checking on things, other people. Oh, and she's got a wheelchair – says she's hurt her back. It's not true: she's perfectly fit. But she thinks something's going on, something sinister as far as she's concerned and so now she's decided to tell me the truth.'