The Tinsleys were ordinary people in an ordinary house, and Mona was, by every contemporary description, a bright and friendly child. The previous autumn the family had taken in a lodger introduced to them by Lilian's sister, Edith Grimes of Sheffield, where the same man had previously lodged with her. He called himself Frederick Hudson. He was popular with the seven Tinsley children, who came to know him as Uncle Fred. The lodging only lasted three weeks, ended when he failed to pay his rent, and the parting was friendly enough that the children kept the nickname. The man's real name was Frederick Nodder, a 49-year-old motor mechanic and lorry driver in Retford with a wife he had deserted years before and an affiliation order out against him for an illegitimate child, which is why he had taken to using an alias. None of this the Tinsleys knew. They had simply taken in a man they thought was respectable enough, and their daughter had been polite enough to greet him by name when she saw him outside her school.

What is known of Tuesday 5 January is pieced together from witnesses who came forward in the days that followed. A neighbour of the Tinsleys saw Nodder loitering on a street corner near the school, staring toward the entrance, on the afternoon Mona was due to leave. A bus conductor named Charles Reville, who worked the 4.45 pm Newark to Retford service, remembered a girl matching Mona's description boarding his bus in the company of a middle-aged man. The man bought a return ticket for himself but only a single half-fare for the child, and the two got off at Grove Street in Retford. A passenger named Stanley Betts independently confirmed what Reville had seen. The following day, two of Nodder's neighbours in the village of Hayton saw a young brunette girl in a blue dress standing in the back doorway of his house, called Peacehaven, watching Nodder dig in his garden. By the time police caught up with these statements and arrested Nodder late on 6 January, Mona was already beyond reach.

Mona's parents waited. So did her brothers and sisters. So did most of Newark. The local force searched Peacehaven, drained five miles of the Chesterfield Canal, and combed every drain, ditch, pond, and cesspit within three miles of Nodder's house. After three weeks of finding nothing the Chief Constable called Scotland Yard. Chief Inspector Leonard Burt and Detective Sergeant Skardon arrived on 25 January and expanded the search in what became one of the largest manhunts in British policing to that date. Inside Peacehaven officers found one of Mona's handkerchiefs, scraps of paper with a child's drawings and handwriting, and her fingerprints on the kitchen crockery. Nodder was charged with abduction on 10 January because the prosecution could not yet charge him with murder; the no body, no murder principle then ruled English law. The trial was held at Birmingham Assizes on 9 and 10 March 1937. The jury convicted Nodder of abduction in sixteen minutes. Mr Justice Swift sentenced him to seven years' imprisonment with words that hung in the air: There is one person in this court who knows, and he is silent. He sits there and never tells you a word.

On 6 June 1937, a family boating on the River Idle near Bawtry noticed something caught in a drain below the waterline. It was Mona. Her body had been weighted with wood and metal; a rotting sack lay on the bank above her. The location was about eight miles from Peacehaven. The pathologist at the Retford Mortuary determined that she had been strangled before entering the water. Her funeral was held on 10 June at the Methodist Church in Newark where she had attended Sunday School. Several hundred people lined the streets to watch the small coffin pass on its way to Newark Cemetery. Nodder was charged with murder on 28 June, tried at Nottingham Assizes in November, and convicted in 75 minutes. He was hanged at Lincoln Prison on 30 December 1937 by Tom Pierrepoint and Stanley Cross. Mona's case is now studied as the precedent that helped overturn the no body, no murder rule in 1954, when English law accepted that sufficient circumstantial evidence could convict even when no body had been found. Beneath that legal legacy is a smaller and simpler fact: a Newark family, on a January afternoon, lost a daughter who had been about to come home from school.