In the 1973 film "Soylent Green," the secret ingredient is people: processed human bodies.
The film is set in a dystopian 2022, when overpopulation, pollution, global warming, and the collapse of ocean life have created severe shortages of food and clean water. The Soylent Corporation advertises Soylent Green as a nutritious wafer made from high-energy plankton, making it a vital food source for the crowded population of New York City.
Detective Robert Thorn investigates the murder of a Soylent Corporation board member and gradually uncovers the truth. His friend Sol Roth, a former professor known as a "Book," studies oceanographic reports and realizes that the oceans are dying, so they could not supply the plankton claimed by the company.
Thorn eventually discovers that bodies from government euthanasia centers are transported to processing plants and turned into food. The revelation culminates in his famous warning, "Soylent Green is people!" The film was loosely based on Harry Harrison's 1966 novel "Make Room! Make Room!," though the human-food twist belongs to the movie.