A comet, which has not passed by Earth for 65 million years, is again about to swing by the planet. Most people are partying, but a few are concerned that the Earth will be passing through the comet's tail. These include a team of scientists that enter a bunker in the desert just outside of the Los Angeles the night that the comet is to appear.
That same night 18 year-old Regina Belmont (Catherine Mary Stewart
) is working as a theater usher. Instead of going home after the last performance she calls her stepmom, Doris (Sharon Farrell
) and tells her she will be out all night at the observatory with her science class. She really intends to spend the night with her semi-boyfriend, Larry (Michael Bowen
), having sex in the steel-lined projection room. Larry, the projectionist, is waiting for someone to return a film he loaned them so they could make a pirated copy.
Back at home Regina's 16 year-old cheerleader sister, Samantha (Kelli Maroney
) gets in an argument with Doris accusing her of sleeping with a neighbor while her Sam's father is away with the military in Honduras. Doris decks Samantha and she runs away from the house.
The next morning Larry and Regina awake to find that the film has not been returned. Larry intends to go and get it while Regina stays at the theater to let him back in. While she waits she plays an arcade game and manages to beat the score of some unknown person with initials "DMK", who had topped her previous high score.
When Larry leaves the back of the theater he is attacked and killed by a zombie who drags him off to eat him.
Regina goes looking for Larry and is surprised to see his motorcycle still sitting there. She is then attacked by the zombie, but her father has given her self-defense training and she is able to get away, jump on the motorcycle and take off.
On the way home she sees that there is no traffic on the roads and no people on the sidewalks. Only clothes and red dust. The sky is also a strange shade of red. She begins to realize that something bad has happened. On the street at her home she again sees that there is nothing around but empty clothes and red dust. She is surprised to find her sister is alive, however, still dressed in her cheerleader outfit from yesterday. Samantha, who doesn't realize that anything is wrong yet, tells Regina that she ran away last night, but remembered she didn't have any place to go, so she spent the night in the steel garden shed.
Samantha has a hard time accepting that all the other people on Earth have been turned to dust by the comet. Regina speculates that the steel of the projection booth and the shed somehow protected them. The girls turn on the radio and find that the local station is still operating, so they jump in a car and head into the city to see if anyone is there.
They find that the station is completely automated. They also find they have company, however. Hector Gomez (Robert Beltran
) is a trucker who spent the night in the back of his steel trailer with a girl he picked up. When the two arrived at the city and saw it was deserted , the girl panicked, ran off and was eaten by some of the zombies. Hector has also came to the radio station to see if anyone survived.
Samantha gets on the air and asks anyone who is still alive to phone the station. She soon gets a call from the scientists at the bunker. They advise her that the comet has caused most of the population to turn to dust. People who only got partial exposure will turn to zombies before they succumb to the same fate. They tell her to stay where she is and they will come and rescue them tomorrow.
Hector stays the night but tells Regina that he needs to go to his parent's house in San Diego to see if they survived. He promises he will return.
At the bunker Carter (Geoffrey Lewis
), the lead scientist, argues with Audrey (Mary Woronov
) the head doctor. She complains that they should not be bringing outsiders into the bunker. Carter replies that under the circumstances they do not have a choice. As they watch a helicopter arrives carrying two survivors, an elementary age boy and girl wearing PJs.
Samantha and Regina raid local military armory to get some automatic weapons to defend themselves (Luckily their father also taught them how to shoot). Samantha is depressed as it seems that Regina has already locked up the affections of only available man on the planet, Hector. Samantha has also developed a rash, something that occurs every time she feels stressed. To cheer her up Regina suggests they go shopping.
At his parent's home, Hector confirms that his family is dead. He gathers a few keepsakes, but has to leave the house in a hurry when he is chased by a zombfied neighborhood boy.
The girls hit a large department store and have some fun dressing up in fancy outfits they would never have been able to afford a few days before. Unfortunately they are being watched from the security cameras by a group of former stock boys who now feel they own the store. As the head stock boy taunts them from the PA, the others try to grab the girls. This leads to a gunfight. The boys finally capture Samantha, forcing Regina to surrender.
The scientists arrive at the radio station, but find it empty. On a hunch they decide to check the department store.
In the basement the boys have chained the girls back-to-back. The head stock boy removes his sunglasses and we see that the boys have been partially exposed and are turning to zombies. They play a game of Russian Roulette putting a gun to Samantha's forehead and pulling the trigger. If it fires the bullet will travel first through Sam's head and out, then through the back of Regina's skull, killing them both.
Before the fatal bullet is fired, however, the boys are surprised by the arriving scientists who machine gun them down and rescue the girls. The scientists, seeing Samantha itching, assume she has been partly exposed. They do not tell her that, but have her wait with Audrey and another scientist, Oscar (John Achorn
) for Hector's return while Regina is immediately flown back to the bunker.
Audrey injects Samantha with something that causes her to go unconscious. Oscar believes that Audrey has put the girl down to spare her from becoming a zombie. The two argue and Audrey tells Oscar that she no longer wants to be a party to taking survivors back to the bunker. He draws his gun, but Audrey is faster with hers and kills him.
Back at the bunker, Carter tries to question Regina, but she is being uncooperative. In another room we see two nurses with two adult males lying unconscious on tables. They discuss how the survivors are being brought to the bunker so they can be used as a source of uncontaminated blood. To make them produce as much blood as possible they are operated on and made brain dead so they simply become blood making machines. The nurses note that they can get 300cc per day out of the men, but expect that the children will produce much less. They note that Regina, being a young and healthy teenager, should produce a large about of blood.
Hector arrives back at the radio station to find Audrey, who is now very close to going zombie. While she injects herself with a lethal dose of drugs, she explains that the scientists accidently left the vents open on the bunker and received a partial exposure. They are tracking down survivors hoping that if they can get enough uncontaminated blood they can create a serum to reverse the effect.
At the bunker Regina demands to know where her sister is. Carter, who is starting to turn, tells her that Samantha has been euthanized before she could become a zombie. Regina is taken by guards to the medical area to prepare her for becoming a blood donor.
Samantha is fine as Audrey only gave her a sedative. She and Hector arrive at the bunker and Hector knocks out the guard and rigs the trucks parked outside, while Samantha sneaks inside. She finds Regina and they are able to rescue the kids from the nurses just before they were about to start operating. Regina, Samantha and the kids escape and run to the car where Hector is waiting. The scientists attempt to pursue them in the trucks, but Hector has sabotaged them and they blow up.
Back in L.A. the rain has washed away the last of the dust and the sky has cleared. Hector, Regina and the two kids have become a family unit. Regina is taking pictures of the them dressed in new clothes. Samantha, dressed like she ready to go to the beach, waits for them, apparently feeling left out. Mocking her sister for not letting the children cross against the stoplight in the deserted city, Samantha is nearly run over herself by a racing sports car. It is driven by a handsome teenager who apologies and tells her his name is Danny Mason Keener (Marc Poppel
). He invites her for a ride and Samantha is delighted to find a boy her age. As they drive off we see his license plate reads "DMK." The credits roll.