Dance of the Dead

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A high school prom in Georgia is unexpectedly interrupted when a graveyard, next to a power plant, becomes the sudden source of resuscitated cadavers. As zombies march on the high school, a motley group of dateless teenage outcasts, among them Jimmy (Jared Kusnitz), Lindsey (Greyson Chadwick), Steven (Chandler Darby) and Kyle (Justin Welborn), take on the zombies and save the day.
Lindsey breaks up with Jimmy the day before the prom and starts going out with Mitch. On the way to the prom, Mitch takes Lindsey to the cemetery to "loosen up" and makes out with her. Mitch is drawn out of the car and killed by a zombie then turns into one himself. Meanwhile, members of the school's SciFi club, Jules, Steven, Rod and George, are investigating the cemetery when they are attacked. Rod is killed and the remaining three are rescued by a grave digger who reveals that he was aware of the occurrences but kept it quiet to keep his job. He instructs them on how to kill the zombies before running off. The three get surrounded again, but Lindsey comes to the rescue and they all escape in Mitch's car.
After making a pizza delivery and discovering that a family are now zombies, Jimmy finds a crashed truck with his enemy Kyle as the only survivor. Jimmy and Kyle run into a cheerleader named Gwen, and the three are attacked. They manage to kill all of the zombies with a bat they find and a gun Kyle carries in his truck. Unfortunately Jimmy's truck is stolen by two zombies, leaving the three without a getaway vehicle. In another neighborhood, Nash Rambler (Blair Redford), Jensen and Dave the Drummer are in the middle of writing a new song for their band. Nash asks Jensen to open the garage door to let in some air because Jensen has been smoking pot, and they discover zombies when Jensen opens the garage door. The zombies attack, but the band members also accidentally find that the zombies like music, and they keep playing to keep the zombies at bay, but also inadvertently attracting more.
Mitch's car breaks down, and Lindsey, Jules, George and Steven take refuge in a nearby house. Jimmy calls Lindsey to find her. He, Kyle and Gwen then make their way to the house through the town sewers. On the way, they discover a substance that the power plant dumped that they believe caused the re-animation of the dead. The house in which their friends are hiding turns out to be a funeral home. Fumes from the power plant re-animate the corpses stored there. Kyle and Jimmy kill them all, but not before Kyle is bitten in the neck and turns into a zombie and the others are forced to beat him to death. Gwen manages to retrieve the funeral home's hearse and the group escapes with the intent to rescue their fellow students from the prom. On the way to the school, the hearse's tires are shot out. The teens also discover their coach is still alive, and after telling him what has happened, he helps arm them. The group heads for the prom in the coach's Hummer. On the way they find and rescue the besieged band. The band then joins the group to stop the zombies.
At the school they find all of the town's zombies have gathered there and that they've arrived too late to save everybody. The coach decides to use his explosives to blow up the school. While the coach starts setting up around the school, the schoolkids block all the doors so the zombies can't escape. While looking through the school, the SciFi club members find a small group of survivors, including the prom queen, and start to lead them out to safety.
The coach accidentally drops the detonator into a chip bowl. Jimmy, however, is forced to go back for it, and Lindsey follows him. They sneak through the gym to get to the detonator but are noticed and attacked. Before they can be killed, the onstage band starts playing to distract the zombies. Jimmy and Lindsey dance while looking for the remote.
On the way out the others are attacked but manage to kill their attackers, while Gwen hides the fact that she was bitten. She pulls Steven, for whom she has developed feelings, into the bathroom. There she reveals the truth and the two kiss, but she turns into a zombie and bites Steven's tongue off. She kills him, which turns Steven into a zombie as well. The two zombies kiss again and then start to eat each other.
In the gym, Jimmy and Lindsey are still looking for the detonator when a zombie accidentally pulls the plug on the band's instruments, stopping the music and causing the zombies to attack again. While the two hold off the zombies, Jenson tries to replace the plug. Jimmy and Lindsey are forced to hide beneath the bleachers, where they are attacked by Mitch, now a zombie. The two escape with Lindsey killing Mitch. The band manages to plug in their instruments and begin playing, allowing Jimmy to get the detonator. As the group attempt to make their escape, Jensen is caught and killed by the horde. The surviving four make their way out of the school through a window. The zombies are unable to escape out the blocked doors, and Jimmy blows up the school, killing all of the zombies, while kissing Lindsey at the same time rekindling their relationship.
The survivors, including a group that hid at the prom, takes a bus to a pancake house for breakfast courtesy of the coach with a plan to eat and plan an attack to shut down the power plant and prevent the zombie plague from spreading. The grave digger also survives and complains about the fact he has to clean up the mess.

Question: Why did Jimmy go back?
Answer: to get the detonator