Plot
Cellular biology professor and former US Army soldier Lena is under interrogation. She recounts her expedition to an anomalous zone known as the Shimmer.
The Shimmer began expanding three years prior from a meteor that crash-landed at the St. Marks Light lighthouse in the St. Marks National Wildlife Refuge in Florida. Expeditions into the Shimmer invariably result in the complete disappearance of the crews. Lena's husband Kane disappears on one such expedition, but inexplicably appears inside their home one year later. Kane's condition quickly deteriorates, so Lena calls an ambulance, but security forces intercept them and take them both to a secret facility. Kane is put in intensive care, and psychologist Dr. Ventress invites Lena to join her on a new scientific expedition into the Shimmer.
Ventress and Lena join forces with three other women: Cassie, a geomorphologist; Anya, a paramedic; and Josie, a physicist. After they enter the Shimmer, they suddenly find themselves several days' journey into their expedition with no memory of what happened. Their communication equipment fails. They witness wildlife with bizarre mutations, and Josie is attacked by an albino alligator with several concentric rows of teeth.
Taking refuge at an abandoned military base, the group finds a video message recorded by Kane's earlier expedition, in which Kane cuts open another soldier's abdomen to reveal slithering intestines. The group finds the soldier's corpse, which has turned into an overgrown colony of lichens. At night, Lena and Ventress reflect on humanity's strange instinct to destabilize itself. A mutant bear attacks the base and drags Cassie away. Lena later finds her mutilated corpse.
The survivors reach an abandoned village full of plants that have taken on humanoid shapes. Josie theorizes that the Shimmer distorts and transforms everything within its boundaries the way a prism refracts light — including the expedition members' own DNA. Anya is overcome with paranoia after watching her fingerprints change. She ties the other team members to chairs, and accuses Lena of murdering Cassie. The mutant bear returns and lures Anya away by emitting a cry for help in Cassie's voice. The bear kills Anya, while Josie frees herself and shoots it.
Ventress reveals she has cancer and is nearing death. She leaves the group and heads for the lighthouse at the center of the Shimmer. Josie believes the Shimmer "refracted" Cassie's dying mind into the bear, and laments how only Cassie's pain and fear remained. She allows herself to succumb to the Shimmer and "refract" into a humanoid plant to avoid a similar fate. Lena follows Ventress to the lighthouse, where she discovers Kane's remains and a videotape. In the footage, Kane instructs someone to find Lena before killing himself with a phosphorus grenade. After the explosion, a Doppelgänger of Kane steps into frame.
Within the hole created by the meteor, Lena finds Ventress, who explains that the Shimmer will swallow everything, and then she disintegrates into a shimmering cloud. The cloud absorbs a drop of blood from Lena's face and changes into a faceless humanoid that mimics Lena's movements. Unable to outrun the creature, which is slowly transforming into her Doppelgänger, Lena tricks it into igniting one of Kane's leftover grenades. Lena flees the burning lighthouse, and the Shimmer dissipates, destroying itself.
Back in the present, Lena's interrogation concludes. She learns that after the Shimmer fell, Kane's condition began to stabilize. Lena visits Kane and asks if he is really Kane. He asks if she is Lena, but she does not answer. They embrace and their irises shimmer.
