The Eliza Armstrong case was a major scandal in the United Kingdom involving a child bought for prostitution for the purpose of exposing the evils of sexual slavery. While it achieved its purpose of helping to enable the passage of the Criminal Law Amendment Act 1885, it also brought unintended consequences to W. T. Stead.

Since the middle of the 19th century, efforts by the Social Purity movement, led by early feminists such as Josephine Butler and others, sought to improve the treatment of women and children in Victorian society. The movement scored a triumph when the Contagious Diseases Acts were repealed under pressure due to their double standard nature and ultimate ineffectiveness.

At the same time, the campaign had also turned towards the problem of prostitution, and male oppression of women. By the end of the 1870s, this had become particularly focused on fears that British females (women and children) were being lured—or abducted—to brothels on the Continent, especially since this was happening to children and women without their consent. Although the age of consent was raised to 13 when amendments to the Offences against the Person Act 1861 were made in 1875, the movement sought to further raise this to at least 16, but Parliament was reluctant to make this change.

However, a Criminal Law Amendment Bill to change this was introduced in 1881. While it passed the House of Lords easily in 1883 after a two-year Select committee study, it stalled twice in the House of Commons. Then in 1885, it was reintroduced for a third time, but again it was threatened to be set aside ultimately because of a political crisis and the upcoming general election that year.

Parliament recessed for the Whit Week bank holiday on 22 May, and upon the following day Benjamin Scott, anti-vice campaigner and the Chamberlain of the City of London, went to see W. T. Stead, editor of the Pall Mall Gazette. Stead was a pioneer of modern investigative journalism, with a flair for the sensational. He was a supporter of the Social Purity movement.

Scott told stories of sexually exploited children to Stead, who agreed to work for popular support. Stead set up a "Special and Secret Committee of Inquiry" to investigate child prostitution, which included Josephine Butler, as well as representatives of the London Committee for the Suppression of the Traffic in British Girls for the Purposes of Continental Prostitution (of which Scott was the chairman) and the Salvation Army. As part of the investigation, two women, an employee of the Pall Mall Gazette and a girl from the Salvation Army, posed as prostitutes and infiltrated brothels, leaving before they were forced to render sexual services. Butler spent ten days walking the streets of London with her son Georgie, posing as a brothel-keeper and a procurer, respectively; together they spent a total of £100 buying children in high-class brothels. Stead, in turn, also spoke to a former director of criminal investigation at Scotland Yard to get first-hand information; he later cast his net wide to include active and retired brothel keepers, pimps, procurers, prostitutes, rescue workers and jail chaplains.

Stead felt that he needed something more to make his point: he decided to purchase a girl to show that he could do it under the nose of the law.

With the help of Josephine Butler and Bramwell Booth of the Salvation Army, Stead got in touch with Rebecca Jarrett, a reformed prostitute and brothel-keeper who was staying with Butler in Winchester as an assistant. Although Butler had no problem with Rebecca's meeting Stead, she did not know Stead's reason for doing so.