The film begins in a distant future on the icy planet Niflheim, where we find Mickey Barnes (Robert Pattinson) in a precarious situation. Having fallen from a great height, he survives against the odds. His partner, Timo (Steven Yeun), spots him but chooses not to intervene, believing that Mickey, as the 17th clone, is doomed. As Mickey’s voice echoes his feelings, he expresses his disdain for dying.
To understand how Mickey ended up in this dire place, we journey back over four years to Earth, where he and Timo are entangled with a ruthless loan shark named Darius Blank (Ian Hanmore). To evade fatal repercussions for their debts, they enlist in a mission to colonize Niflheim due to the deteriorating conditions on Earth. This journey leads them to the dominion of Kenneth Marshall (Mark Ruffalo), a celebrity-like figure with a devoted following. In this new life, Timo becomes a pilot while Mickey takes on the role of an Expendable, undertaking perilous tasks including collecting samples of a dangerous virus in search of a vaccine. He grapples with haunting memories of his past, particularly guilt surrounding his mother's fatal car accident, which he suspects he inadvertently caused.
Amidst the challenges of the colony, Mickey forms a bond with Nasha Barridge (Naomi Ackie), a security agent. Their connection deepens despite the strict regulations imposed by Marshall, who explicitly forbids any sexual relationships among colonists—a rule Mickey and Nasha frequently disregard.
As Mickey's journey continues, he embarks on an expedition into Niflheim alongside Timo and fellow agents Kai Katz and Jennifer Chilton. Their mission, ordered by Marshall, is to capture a mysterious creature known as a “Creeper.” However, when chaos ensues during their attempt—leading to Jennifer's unfortunate demise—Mickey finds himself down in a crevasse. Unexpectedly, a mother Creeper saves him, using her tail to guide him out, leaving him bewildered.
Upon returning to his quarters, Mickey is alarmed to learn that another version of himself, Mickey 18, has already been created, as they believed Mickey 17 was dead. This revelation sparks panic within Mickey 17, as doubles are strictly prohibited following a notorious incident involving a serial killer who exploited the cloning technology. This precarious situation only escalates as Mickey 18 learns of his existence and becomes hostile.
During a dinner with the Marshalls, Mickey begins to experience severe side effects from a growth hormone included in the meal—thanks to Arkady (Cameron Britton), the lead scientist. Following these events, Mickey tries to navigate the complexities of his relationship with Nasha while grappling with the reality of having an adverse clone.
The tension heightens when Marshall prepares a deadly nerve gas intended for the Creepers, completely neglecting Mickey’s and Nasha’s insights into the creatures' true nature. As the Creepers rise to the surface, seeking to reclaim their lost young, Mickey is faced with a moral dilemma regarding the fate of both the Creepers and humanity.
In a climactic confrontation involving Nasha, the Marshalls, and the Creepers, alliances form and betrayals unfold. Mickey ultimately sacrifices himself to save the Creepers, leading to a chaotic finale. The aftermath sees Nasha ascending to power and Mickey coming to terms with his past while grappling with nightmares of the villainous Ylfa's (portrayed by Toni Collette) influence.
Months later, a significant ceremony marks the shut down of the Expendable program. Mickey, now embracing his true identity, has the honor of destroying the clone printer, symbolizing a poignant ending to a tumultuous journey filled with moral quandaries, transformations, and a quest for redemption amidst cloning's chaotic consequences.
