In 1982, a Soviet moon lander crashes into the frigid Bering Sea, setting off a mystery that will resurface decades later. Fast forward to 2015, and biology professor Stephen and his graduate students Ronelle and Sadie join an experimental sea voyage to study how warming oceans affect whale populations. They ride out with Sadie’s grandfather, Graff, aboard his weathered fishing trawler, the Harbinger, which is crewed by the sturdy hands of Bowman, Roland, Big G, Dock, Svetlana, and Atka. The journey blends scientific curiosity with the rough-and-tumble life at sea, where every creak of the hull seems to carry a new clue.
One night, Sadie is roused by the crew’s activity and the ship’s sonar picks up anomalous whale signals in the area. She awakens the others and, with their help, she helps locate and recover the ice-encased lander. The discovery triggers a conflict: Stephen immediately asserts ownership, but Graff counters that the find should belong to Sadie, who discovered it first. Bowman pushes Sadie to investigate before Stephen can seize the discovery for his prestige or career. As Big G distracts Stephen, Sadie carefully examines the lander and uncovers the body of a cosmonaut, who apparently died from an unknown infection, and she collects a skin sample to study later. The tension rises when Stephen learns that Sadie had already examined the lander, and he threatens to ruin her career unless she signs over the discovery to him. Graff warns of legal action, but Sadie finally yields. She soon learns the lander carried tardigrades, hardy organisms capable of surviving extreme conditions; but a cosmic radiation exposure has mutated them into something far more dangerous and capable of shapeshifting.
When the cosmonaut’s body goes missing, Stephen’s curiosity turns to contamination as he examines the lander and becomes infected by the tardigrades. The reanimated cosmonaut’s body kills Roland, and Stephen, chilled to the bone, strips on deck in the sub-zero air before the others pull him back inside. Dark, lifelike growths sprout from his back and spray a murky liquid over the crew. Sadie confirms that the entire party could be contaminated. The group freezes Stephen’s body and the lander using liquid nitrogen to halt their spread.
As Dock begins showing signs of infection, Svetlana locks him in a cage and plans to incinerate him, but the situation escalates when Dock also sprouts tentacle-like growths. The ship’s power is knocked out, and the backup battery surges to life just as the crew fights to stabilize the vessel. The organism proves deadly again when it kills Atka, and Svetlana’s true allegiance is revealed: she is a Russian spy. She explains that the tardigrades were part of a Soviet experiment to create radiation-resistant cosmonauts and reveals a plan to sink the Harbinger after they board a nearby Russian submarine. Before she can carry out her plan, the organism lashes out, dragging Ronelle and Svetlana away.
In the wake of the attack, Graff, Sadie, Bowman, and Big G search for Svetlana’s hidden explosive charges scattered around the boat. The last of the charges lie in the cramped bilge, a space Sadie alone can access. As they collect the ordnance, the others learn that the tardigrades have eaten and absorbed their last catch—two tons of crab—heightening the danger. Sadie realizes she is now standing in the midst of a living swarm and narrowly escapes, but Graff becomes infected. He urges Big G to restart the boat and asks Sadie to take the helm, then tells Bowman to kill him to prevent further suffering.
Big G attempts the restart but is killed by the mutated Svetlana. Before Bowman can freeze Graff with liquid nitrogen, the creature strikes, impaling Bowman. Graff, in his final act, urges Sadie to scuttle the ship and save herself. She steers the Harbinger toward an approaching iceberg and escapes, while the organism bursts through the deck behind her. Through a portable radio, the US Coast Guard relays that a rescue helicopter is closing in. Sadie, exhausted and battered, collapses onto the ice as the rotor blades shimmer on the horizon, and she slips into unconsciousness, the Harbinger’s peril finally giving way to a fragile hope of rescue.
