Lee Dae-ho (Kim Sang-kyung) is a single father and manager of the 120-story luxurious landmark building complex, Tower Sky in the business district of Yeouido in Seoul. He is an earnest employee liked by his colleagues and is secretly in love and secretly even wanted to marry with Seo Yoon-hee (Son Ye-jin), a restaurant manager.The owner of the complex, Mr. Jo decides to hold a "White Christmas" party for tenants and VIPs on Christmas Eve, with helicopters circling above with huge lights attached below them, sprinkling snow onto the party. Dae-ho has promised to spend the day at an amusement park with his daughter Ha-na, but has to cancel when he is needed at the party. However, the building has faulty water sprinklers due to frozen pipes, but Mr. Cha, the Tower Sky's safety section head, is more concerned for the party than any possible architectural errors within the building, despite warnings from his fellow employers about these errors.One example of their warnings is when cook Young-cheol (Jeon Bae-soo) accidentally leaves a stove on for too long and causes a minor fire. Young-cheo is in love with a receptionist within the building named Min-jung, and even sneaks out of the kitchen while on duty to make her an ice cream.Meanwhile, Lee Seon-woo (Do Ji-han) is a rookie fireman entering the Yeouido Fire Station. When he gets accepted, he learns that many of the firefighters get more breaks than actual firefighting. In a prank to fool Seon-woo, the other firefighters ring the fire alarm for the squad to assemble while he takes a shower, causing him to appear in front of his entire team naked, much to the other firefighters' and even his own pleasure as they put his helmet on him for the first time.That evening, while the party is in full swing, and while Young-cheo proposes a rose to Min-jung in an elevator, huge gusts of wind cause one of the helicopters to lose control and the attached lights crash into the glass bridge connecting the two Tower Sky buildings and into a side of one of the buildings. Another one of the helicopters then crashes into the building, causing the helicopter to leak fuel and the building catches fire. Shards of glass fall onto the partygoers below, which include Mr. Yoon (Song Jae-ho) and his friend Mrs. Jung (Lee Joo-shil), while another one of the huge lights falls and crashes onto a bus on the street.At one point, while evacuating, some people, including a cowardly security guard, board an elevator despite warnings not to use it, as the security guard rejects Yoon-hee and Ha-na from entering just as they are about to board. While traveling down, an explosion occurs below the elevator, which stops it in its path. This literally fries the people inside while melting their shoes until the elevator explodes, killing everyone in it.Dae-ho moves quickly to save Ha-na, Yoon-hee and his colleagues as they try to find a way to escape. At the same time, firefighters, led by Captain Kang Young-ki (Sol Kyung-gu), as well as Seon-woo, arrive to fight to bring the blaze under control, helping Dae-ho in the process. Together they fight to save the lives of everyone.While Yoon-hee, Ha-na, Young-cheol, Ming-jung and the others take refuge in the building's Chinese restaurant, the firefighters decide to contain the blaze at its origin: the 63rd floor, where the helicopter crashed. Despite containing the fire on that floor, it spreads further within the building.Just when all seems hopeless, Young-ki sacrifices his own life to save not only Seon-woo, but for all people in the future, as he detonates a bomb to demolish the building, in order to prevent it from being damaged any further.
Output: Question: What ignites to start the fire?, Answer: Helicopter fuel.