Many years ago, before I was a novelist, there was a man I worked with who became a friend for a short while. It was a casual friendship and completely platonic - he was married, I had a boyfriend. Looking back now, I realize that our friendship existed during an unhappy time in my life. Dissatisfied with my career, unfulfilled by the relationship I was in, I decided on a radical course of action, quitting my job, breaking-up with my boyfriend, moving out of the home I shared with him - all things that were difficult at the time, but I have never, for a moment, regretted them.

Karen Perry

Karen Perry

A casualty of this life-overhaul was the loss of several friendships. One of them, this man (let's call him X) I soon lost touch with. We were never terribly close, but there was one evening, having a drink together in a bar in Dublin, that he revealed something to me: a secret that he had. He told me he had a child from a previous relationship, a child his wife knew nothing about. It was the kind of revelation that induces shocked silence. You think you know a person, you think you have the measure of them, and then they hit you with something like that. I was floored. How old was this child? I asked, and when he told me the answer - nineteen - I couldn't help but think of this secret as a ticking bomb. It seemed to me that this grown-up child would surely come knocking on X's door any day, looking to find his/her father. How would X explain this to his wife? His responses to my questions were evasive and after a while, it became clear he no longer wished to discuss it.

That was the last time I ever saw X. Perhaps he regretted his admission and sought to avoid me. Perhaps I should have tried harder to stay in touch. Over a decade has passed since, and my life is changed considerably. I am happily married now, and the mother of two wonderful daughters. I have written and published seven novels, three of them co-authored by my good friend Paul Perry (Karen Perry is our pseudonym). It was not until January 2014 that X came into my head again. At the time, Paul and I were trying to complete our second novel together, Only We Know. During the last-gasp stages of writing a novel, I find my energy starts to flag and my attention begins to wander, and it is often at these moments when ideas for the next book announce themselves. I hadn't thought of X in years, but suddenly the memory of our discussion that night came back to me so vibrant and clear. I wondered whether his child had ever made contact? Had his wife somehow found out? I'm intrigued by the secrets people carry around with them, and this one seemed so loaded - almost dangerous. The more I thought about it, the more the desire grew to explore the notion through writing. I asked myself how I would react were I to learn that my husband had a child I knew nothing about? What would I do if I opened my front door one day and found a teenager on the doorstep claiming to be a half-sibling to my two little girls? Would I have to make space for this person in my family? Would they expect to move in and receive the same treatment as my daughters? What if I felt I could not trust them? Had it the potential to tear our family apart?

A lost memory found. The seed of an idea planted. Paul and I spent hours discussing it, the idea growing and forming until it became Girl Unknown - the story of a University professor whose life is turned upside down when one of his students comes into his office and informs him that he's her father. As she becomes ever more entangled in his life and the lives of his family, how can he be sure if this girl really is his daughter or a deadly cuckoo in the nest?

Girl Unknown is out now.