A group of young American adults, Chris (Jesse McCartney), his girlfriend Natalie (Olivia Taylor Dudley), and their mutual friend Amanda (Devin Kelley), are traveling across Europe. They stop in Kiev, Ukraine to visit Chris's brother, Paul (Jonathan Sadowski) before heading on to Moscow where Chris intends to propose to Natalie.After a night on the town, Paul suggests that they go on an "extreme tour" and "off the beaten path" of the abandoned town of Prypiat which sits in the shadow of Chernobyl Nuclear Power Plant, site of the infamous nuclear disaster. Paul briefly explains that on April 26, 1986, Reactor #4 at the plant exploded after suffering a massive meltdown. The explosion took place at 1:23 am while the neighboring town of Pripyat slept. Two workers were killed instantly. The fires were put out a few hours later, but the damage was done as radiation from the destroyed reactor swept across the plant and the entire countryside. Nearly all of the workers and the firemen who put out the fire died from radiation poisoning within a few hours to a few days. 40 hours later, the residents of Pripyat were ordered to evacuate by the Soviet government, and most never returned; by that time, many of the residents had suffered varying degrees of radiation poisoning. (All true)The group is joined by backpacking couple, a Norwegian named Zoe (Ingrid Bolso Berdal) and her Australian boyfriend named Michael (Nathan Phillips).The six-person group is initially denied access to Prypiat by the Ukrainian military because of the lingering radiation emiting from the closed down plant, but their Ukrain tour guide, Uri (Dimitri Diatchenko) takes them by van in through an unmanned, forested check point. On the way, they stop at a river where they discover a large, mutated fish with large teeth dead at the side of the river. As the group returns to the van, a number of similar fish are seen below the murky water attacking a piece of beef jerky Uri threw earlier on.They spend a few hours exploring the abandoned city, which was home to Chernobyl workers and their families before they were forced to evacuate overnight. When Uri takes them into an apartment building, he hears a noise. When he slowly goes to investigate, a bear comes out from an adjacent room and runs away. Frightened, the group leaves the building. Uri comments on the incident, telling the group he has been coming to Chernobyl for five years to give private tours and has always seen a dog or wolves but never a bear.As the group prepares to leave, Uri finds the wires in his van have been chewed through and tries to radio for help to no avail. As night falls, the group begins arguing and affixing blame. Noises are heard outside and Uri goes out with a flashlight and a gun, accompanied by Chris. Shots are heard and Paul runs out to investigate, only to return with Chris whose leg has been severely mauled, seemingly by dogs who attack the van. The groups stays in the van the whole night.The next morning, Paul, Michael, and Amanda go out looking for Uri whom, according to Chris, was taken by "them" while Paul and Zoe stay behind in the van to look after the wounded Chris. They follow a trail of blood to find Uri's mutilated body in an underground complex. They take his gun and radiation Geiger counter and barely escape a group of mutant creatures (semi-human looking nomads whom are scarred by radiation poisining). They return to the van, planning to hike to the nearest checkpoint but Chris cannot move his leg. Chris and Natalie stay behind in the van while the others go to get help.The group comes across a group of modern abandoned vehicles in a parking lot and find a bus with bullet holes exiting from inside it. In the bus they also find broken guns and a bloody guard uniform. Michael finds replacement wires for the van, so they head back toward their vehicle. After being chased by mutant dogs, they reach the van by nightfall and find it ripped apart and flipped over. Natalie's camera video reveals she and Chris were taken by the mutants.They go in search of the two but end up being chased by more mutants. During their escape they find a traumatized Natalie and rescue her. When the group are out in the open, they begin to work out where another checkpoint nearby is by using a map Michael obtained during their previous escape. The group is distracted by a little girl in the distance. As they try to communicate with her Natalie is once again taken by the mutants. Paul, Michael, Amanda, and Zoe attempt to find her, but in the process are swarmed by a horde of mutants. They run to a fallout shelter, but Michael is captured. The three continue on, finding Chris's engagement ring for Natalie but no sign of Chris himself. They are again attacked by mutants and as they begin to climb a ladder to the way out a congregation of mutants pull Zoe back down, forcing Amanda and Paul to leave her behind as they escape.Paul and Amanda continue on through the fallout shelter which comes out in Chernobyl power plant itself. As they navigate around the #4 reactor, they find Natalie dead. They continue on, only to also become poisoned with radiation as their skin begins to blister. They exit the reactor and are confronted by Ukrainian military personnel. Blinded from radiation poisoning, Paul is shot dead when he approaches the soldiers. Amanda is taken away as the rest of the Ukrainian soldiers begin to sweep the area shooting and killing various mutants.Amanda passes out and awakens on a gurney in an unknown location where several Russian and Ukrain doctors, clad in protective hazmat suits, inform her that she is in a hospital. The doctors talk amongst themselves (in Russian, so Amanda cannot understand them) and reveal that the "creatures" were escaped patients from a nearby asylum whom they have placed around Chernobyl for the past 10 years by the Russian, Ukraine and Bellarussian authorities as test subjects to test the levels of radiation effects on them. They doctors note that after conducting the sweep, all of the mutants have been killed or re-captured and the other Americans are dead. They decide Amanda, having seen the escapees, must not be allowed to leave, so they throw her into a dark isolated room. Amanda is immediately alerted as she begins to hear noises. She calls out and a group of mutants attack her.
Output: Question: Who swarms the group?, Answer: Mutants