Masanjia was established in 1956 under China's re-education through labor policy, known as laojiao. The system allowed police to detain people for up to four years without trial. Masanjia sat in the Yuhong district near Shenyang, sometimes called the Ideology Education School of Liaoning Province. The camp's population included petty criminals, people convicted of drug offenses, sex workers, and persistent petitioners who had irritated local authorities. In 1999, following the government's crackdown on the Falun Gong spiritual practice, the camp was significantly expanded. According to former detainees, Falun Gong practitioners came to represent between 50 and 80 percent of the camp's inmates. Members of underground Christian churches were also held there.

Former inmates described a regime of forced labor and systematic abuse. According to interviews published by the New York Times in June 2013, the most sustained and severe treatment was directed at Falun Gong practitioners who refused to renounce their beliefs, though other prisoners experienced abuses as well. Former detainees described methods including the "tiger bench," where a person was seated, tied at the waist, bent forward with hands and feet immobilized, and bricks placed under their feet. The "dead person's bed" involved being spread-eagled on a bed frame for prolonged periods. Wang Chunying, detained at Masanjia in 2007, told the Japan Times she was handcuffed between two bunk beds for 16 hours, unable to eat, drink, or sleep. These were not isolated incidents but documented patterns reported consistently by multiple former detainees across different years.

In April 2013, China's Lens Magazine published a 14-page investigative report based on interviews with approximately 20 former inmates. The 20,000-word story described forced labor and torture methods in detail. It caused an immediate sensation: within two days, at least 420,000 people had participated in online discussions about the report. The next day, the Central Propaganda Department issued instructions prohibiting all news organizations from reposting, reporting on, or commenting on the story. The directive came too late. The report had already reinvigorated national calls to reform the forced labor system. China formally abolished the laojiao system in December 2013, though human rights organizations have noted that similar practices continue under different names.

Sun Yi, the man who hid the SOS letter in a box of Halloween decorations, was eventually released from Masanjia. He cooperated with filmmaker Leon Lee to create the documentary that bore the camp's name. The film was banned in mainland China, and journalist Du Bin, who had also documented conditions at the camp, was detained. Masanjia was built on top of a graveyard, a detail that former prisoners noted with grim irony. The camp's story is a reminder that the products of forced labor can end up anywhere, that a letter hidden inside a plastic tombstone can travel from Liaoning Province to Oregon, and that the people held inside systems like laojiao were not abstractions but individuals who smuggled messages in the only way they could.