In 2022, New York City grapples with a staggering population of 40 million, leading to rampant overcrowding and dilapidated housing. The once-bustling streets are now overflowing with the homeless, while the city's residents are forced to contend with an unemployment rate hovering around 50%. The oppressive heat, exacerbated by severe climate change from the Greenhouse Effect, has made summers unbearably humid, creating conditions where temperatures soar above 90°F, day and night. Food has become a scarce and costly resource, reminiscent of a bygone era. Technology has stagnated due to the depletion of Earth's natural resources, with few trees remaining. Most of the populace relies on processed rations from the Soylent Corporation, including the notorious Soylent Red and Soylent Yellow—both marketed as "high-energy vegetable concentrates." The newest offering, Soylent Green, is touted as being made from "high-energy plankton," presenting itself as a more nutritious option, though it remains in limited supply, leading to weekly food riots.
The story follows Robert Thorn (Charlton Heston), a detective with the New York City Police Department, who resides in a cramped, rundown apartment with his elderly friend and roommate, Solomon "Sol" Roth (Edward G. Robinson). Once a college professor, Roth now dedicates his efforts to sifting through the chaos of what remains of written history to aid Thorn's investigations. Despite Roth sharing tales of the past from a time of abundance and sustainability, Thorn remains skeptical, finding the stories hard to fathom.
Thorn's latest case involves the murder of William R. Simonson (Joseph Cotten), a wealthy lawyer found dead in his lavish apartment at Chelsea Towers West. Upon the scene, Thorn converses with the building superintendent, Charles (Philip Stone), who guides him to Simonson’s residence, where the detective discovers the lifeless body lying in a pool of blood, having suffered multiple blows to the head. Rather than diligently seeking clues, Thorn indulges in the luxuries of the affluent man's food and drink, relishing a shower with real hot water.
During his investigation, Thorn meets Shirl (Leigh Taylor-Young), the attractive 23-year-old "concubine" designated to Simonson, and Tab Fielding (Chuck Connors), Simonson's bodyguard. Fielding asserts he was away during the murder, leading Thorn to distrust his alibi. Afterward, Thorn returns to his apartment, sharing a meal with Roth from the food he pilfered from Simonson’s apartment, where they unravel the victim’s connections to the Soylent Corporation.
Later, Thorn spots Shirl during her gathering with other "furniture" girls, and she reveals Simonson had been increasingly distressed in the days leading up to his untimely death. In the throes of his investigation, Thorn learns from a worn-out priest (Lincoln Kilpatrick) that Simonson's confession contained something so deeply troubling that it remains unspoken, prompting Thorn's further digging. The plot thickens as he unearths evidence that links powerful individuals like Governor Joseph Santini (Whit Bissell) to the case, only to find himself embroiled in a conspiracy that compels him to tread carefully.
As Thorn navigates the violent street riots and corrupt law enforcement, he realizes the stakes are criminally high when Gilbert (Stephen Young), a hired gunman, attempts to eliminate him. An explosive chase ensues, culminating in Thorn discovering the horrifying truth about Soylent Green: its real composition is human bodies, not plankton as the public has been led to believe.
Ultimately, through relentless determination, Thorn races against time to expose the truth. In a heartbreaking denouement, he confronts the realities of his world as he battles against those who wish to maintain the grim status quo. As he faces life-threatening danger and narrowly escapes death multiple times, Thorn's message relays a desperate urgency: "Soylent Green is PEOPLE!! We've got to stop them... SOMEHOW!!!" His revelation is both a personal and societal indictment, showcasing the desperate straits in which humanity finds itself in this dystopian future.
