Plot
Fifty people awaken in a darkened room, arranged in two concentric circles around a black dome. When they attempt to move from their designated platforms or touch the others, alarms sound off. When someone ignores the warning and leaves formation, a beam from the dome kills them, and their body is quickly removed. As the others panic, a man attempts to calm them, but the device kills him mid-sentence. Thereafter, every two minutes, another person is killed. After several people die, the group realizes that the room's technology allows them to use hand gestures to vote for who dies, while arrows on the floor show each person their own vote but not others'. They attempt to boycott the vote, but someone in the circle is randomly selected to die after two minutes.
Following someone's suggestion, the group buys time to think by deciding in advance to eliminate the elderly for the next selections. The captives discuss where they are, how they got there, who has abducted them, and why. A young man, Eric, remembers attempting to flee Los Angeles, and others concur. Eric says that he was pulled into the air, later waking in a red room with other humans. The old man next in line agrees, saying he saw and heard aliens. The disbelieving group eliminates him instead of listening.
After another member of the group aggressively targets a 52-year-old cancer survivor over the objections of people who do not consider her elderly, he is eliminated. Several people say they recognize the others in the circle: a man identifies the woman next to him as his wife, another man identifies the doctor he was having an affair with, and a tattooed man is eliminated after he admits to a cop's accusation of domestic violence. After several people are quickly eliminated, a black man claims the process has become racist. Several others dispute this, but when the cop goes on a racist rant, he is selected next.
The captives experiment with voting, find they cannot vote for themselves, and attempt to give one vote to every person in the circle. One man gives a second vote for a pregnant woman, so Eric votes for him; this causes a tie and the man is killed in a run-off vote.
It is here that Eric makes the crucial observation that, since they can't vote for themselves, the game will proceed until there is only one player standing, whom will then get to live.
This revalation causes a schism between those who want that survivor to be themselves, those who believe either the pregnant girl or the child–Katie–should get to live, and those who simply withdraw and refuse to kill to stay alive in the circle.
After several of the latter group take their own lives by leaving formation to buy the others time, an atheist antagonizes the theists who praised the volunteers' faith. The atheist is briefly saved, but when he mocks the girl beside him for having her boss pay for her breast enlargement, he is killed. A homophobic lawyer targets a lesbian, and he is killed as a result. The group realizes that one of the final two people left must not vote (and be killed) to render a winner.
The group continues to debate the issue of who should deserve to live, based on factors like morality, ability to contribute to society, or believing that all of their lives are of equal value, or inequal value (the girl having many years left to live, and the pregnant woman counting as two people). When several players make their intentions clear, blocs form.
One bloc (led by Eric, a Marine, and the cancer survivor) believes that everyone should sacrifice themselves to save the pregnant woman and the little girl, while the other bloc (led by a bearded man and a rich man) wants to eliminate them immediately as a threat to their survival, as they believe everyone is equal and no special privileges should be afforded. The husband and wife team initially vote with the bearded man's bloc but the husband is forced to vote with Eric's bloc when they threaten to eliminate his wife. Under interrogation, the couple admit that they concocted the relationship to curry favor, resulting in the elimination of the "husband".
Eric's faction incurs heavy losses but, through manipulation of the few voters remaining and the trust Eric and the two girls share, manages to eliminate the other faction, leaving only four: Eric, the pregnant woman, the girl, and a silent man who has never voted and has remained neutral throughout. The three, with regret, vote the silent man off, who takes his death in stoic silence.
Eric theorizes that aliens have used the process to learn about humanity's values. He tells the girls that in order to ensure the survival of one of them, someone must volunteer with him. The little girl, Katie, heroically decides to sacrifice her own life, that the woman and her unborn child might live.
But it is all revealed to be a trick. As the girl steps off her platform, expecting Eric to do the same, he instead votes for the pregnant woman, ending the round and killing the pregnant woman. A split second later, Katie's foot, already in motion, lands off the platform, and she dies.
Eric is victorious, but when nothing happens, he believes the machine will kill him anyway. Instead, it reveals that there has been a fifty-first player this whole time: the unborn child, still alive in the pregnant woman's now dead body. Eric votes to kill the child, becoming the lone survivor.
He awakes in Los Angeles where he joins a group of people that are as varied as the contestants were in ages and ethnicities - including pregnant women. It is unclear whether or not these are survivors, or the victors of their own circles. Together, they watch the fleet of alien craft floating over the city.
