Located on the southeastern tip of the Korean Peninsula, Haeundae-gu of the city Busan draws one million visitors to its beaches every year. Several years ago, Man-sik, a Haeundae local, lost Yeon-hee's father due to his mistake in the 2004 Indian Ocean Earthquake while deep-sea fishing together at the Indian Ocean. Because of this, for years, Man-sik wasn't able to tell Yeon-hee, his long time girlfriend who runs an unlicensed seafood restaurant, that he loves her (Yeon-hee doesn't know that her father was killed due to Man-sik's mistake.).Meanwhile, Dong-choon, another Haeundae local and also a neighbor of Man-sik, teams up with Man-sik's son and does stupid things to earn money, but the two ends up caught by the police. Afterwards, Dong-choon's mother, concerned about her unemployed son, decides to prepare him for a job interview, but Dong-choon does not want to do the job interview. As time pass, Man-sik finally plans to propose to Yeon-hee on a night where there is are fireworks.Geologist Kim Hwi somehow runs into his divorced wife, Yoo-jin, who is preparing for an expo in Busan. Although Yoo-jin has a daughter (and also a new husband), they decide not to tell their daughter that he (Hwi) is her real father, afraid that the child would be greatly shocked. Meanwhile, a wealthy college student from Seoul, Hee-mi, accidently falls into the ocean from a luxurious yacht. Hyeong-sik, Man-sik's younger brother and also a lifeguard, rescues Hee-mi rather 'violently'. Later on, Hee-mi becomes greatly angered by the 'violent' rescue and annoys him at no end by following him around. But as time passes on, love starts to grow between them.During the fireworks, Man-sik proposes to Yeon-hee. Yeon-hee says that a ribbon hung outside her restaurant the day after would mean 'yes'. On the day after, Man-sik sees no ribbon, and he meets Dong-choon, who was nearby. Dong-choon tells Man-sik that he told Yeon-hee why her father died several years ago. Man-sik becomes very angered, for he thinks that what Dong-choon told Yeon-hee caused there to be no ribbon.Meanwhile, Hwi notices that the Sea of Japan (East Sea) is displaying similar activity to the Indian Ocean of the time of the 2004 Indian Ocean earthquake. The Disaster Prevention Agency assures him that Korea is at no risk, but a megatsunami soon forms near Japan and starts to travel at 500 miles per hour towards Haeundae. Hwi notices that the citizens of Haeundae has only 10 minutes left to escape. There is a short earthquake in Haeundae; the elevator Yoo-jin is in stops perhaps due to the earthquake. People run for their lives, but the tsunami soon reaches Haeundae. The streets are shown quickly flooded while the people are trying to escape, and some buildings collapse. Some people and cars on a the Gwangan Bridge are swept away by the tsunami. A powerful electric shock from a telephone pole happens in the now-flooded street where Man-sik and Yeon-hee is in, but the two survive.A big cargo ship gets stuck at the Gwangan Bridge. Dong-choon, who is on the bridge, tries to escape from the falling cargo containers (from the ship). The containers are shown falling at an extreme momentum. Dong-choon has survived this, and later on, he tries to light a cigarette while still on the bridge. His portable lighter ends up on a flammable liquid flowing from one of the containers; there is a gigantic explosion, where the bridge gets cut in half.Hyeong-sik jumps down from a rescue helicopter and saves Hee-mi, who was far away in the sea with one of her friends. When Hyeong-sik and the friend are together on the rope, Hyeong-sik realizes that the rope is about to break and only one can go up to the helicopter. After a deep thought, Hyeong-sik cuts down the rope his body is connected to and falls down into the violent sea. The elevator Yoo-jin is in gets flooded with water, and she talks with tears to her daughter by her portable phone. Soon, Yoo-jin is saved by a worker. On the roof of the building, Yoo-jin meets her daughter and Hwi. The two helps their daughter go onto a crowded rescue helicopter. Before the helicopter leaves, Hwi tells the truth to his daughter that he was really her father. Yoo-jin apologizes with tears to Hwi, and they hug onto each other in front of the second tsunami.After the tsunami, there is a funeral for the lifeguards who were killed in the tsunami; among the ones who were killed is Hyeong-sik. Dong-choon realizes that his mother has passed away in the tsunami, and he breaks into tears. People are shown helping reconstructing the city. Man-sik, while cleaning the ruins of Yeon-hee's restaurant, finds the red ribbon which Yeon-hee's said was a 'yes' for his propose. The movie ends with the scene of Haeundae in ruins, but rather in a hopeful atmosphere.
Output: Question: Why does the police arrest Man-sik's son?, Answer: Man-sik's son does stupid things to earn money