This is a world much like ours. Completely identical in fact, with one very specific, very important difference.
Nobody knows quite how and why it happened. If it started at some point in the past, that point in time was before recorded history. Whether it was magical or scientific or something else none can tell, but magic probably has something to do with it. It does not appear to be genetic and is completely random - all the members of a family might change, or one identical twin may change and their sibling will not.
It's come to be called second puberty, among other euphemistic names such as the Event. It affects about 5% of the human population. What happens is that, starting after a person's 18th birthday, they may undergo a change into a member of the opposite sex. The process usually takes a few months, but extremes of a few weeks to a few years are not unknown. And while it is usually soon after the 18th birthday, sometimes it is delayed as far as the late twenties. No cases have been recorded after the age of 30.
Second puberty is different for everyone who undergoes it. Sometimes it is painful. Sometimes it is pleasurable. Sometimes it is barely noticeable. The changes can be minor - simply adding functional genitals and secondary sexual characteristics without any further change - or drastic. Body type and shaped may be retained or altered. Mass may be gained, preserved - a chubby girl finding herself transforming into a tall, slim man of the same weight - or lost - a bulky athlete finding bone and muscle converted to fat and then rapidly burned as he shrinks into a petite woman. The changes can happen in sequence or all together, out of sync or in perfect unison. Either way, the end result is a man or a woman who is indistinguishable in every way from a person born of that sex.
Second puberty is a time of changes not only in appearance but in personality and perception as well. Learning to deal with a new body is difficult. Extroverts may become shy, shrinking violets may become boisterous. If the Event happens late in life, it can cause all sorts of problems - a happy parent of three children changing sex can break a home, or at the very least change it significantly. The worst cases take their own lives in anguish, unable to handle the massive changes to themselves and the way others see them. Some cultures view it as something to celebrate, others as something to be ashamed of and hidden when it happens until it is complete. There have been books written, movies made, and countless therapies and coping mechanisms devised to help Changers through this difficult moment in their lives.
Our story starts as we take a closer look at someone...