Between 1912 and 1914 three brides drowned in their baths, in three towns, with three verdicts of accident. Nobody connected them until a grieving father opened a Sunday paper and recognised the shape of his daughter's death.

In 1983 a 16-year-old walked to her boyfriend's house in an English village and never arrived. Her murder was the first case ever shown on Crimewatch. 25 years later, a traffic stop of a young man not yet born in 1983 gave the police a name.

The biggest manhunt Britain had ever mounted spent 18 months chasing a man who did not exist. It took 26 years and a scrap of dried envelope glue to find out who invented him.

In 1992 Rachel Nickell was killed in broad daylight on a London common while her 2-year-old son watched. For 16 years the police chased the wrong man - while the real killer was already in their files.

The same female DNA turned up at 40 crime scenes in three countries, including the murder of a young policewoman. Hundreds of officers hunted her for years. Then one impossible test result revealed where she had really come from.

Two women left a house in Lancaster in September 1935 and were next seen as parcels in a Dumfriesshire stream. The killer had removed everything that identifies a body. Then a team of anatomists put the bodies back together.

A hunter found a steel drum in the New Hampshire woods in 1985. Inside were a woman and a child. It took four decades, a new science and a librarian's hunch to give all four victims their names back.

On Valentine's night 1988 a twenty-year-old woman was stabbed to death in a flat above a Cardiff betting shop. Three innocent men went to prison for it. The man who did it lived quietly two miles away for fifteen years, until a scientist scraped back the layers of paint on a skirting board.

A fifteen-year-old took a shortcut home in 1983. The investigation that followed produced the world's first DNA exoneration, the first DNA murder conviction, and a mass screening that never found the man who did it.

On a January morning in 1974, three teenagers came home from school and found their parents and their two youngest siblings dead in the house. For three decades the man who did it walked around their city as a church council president. He was caught because he could not stop writing to the police.

For four decades he was three different monsters with three different names, and the men hunting him could not agree that he was one person. The break, when it came, did not come from a witness or a confession. It came from strangers who had spat into a tube to find out where their grandparents were born.