Located on the southeastern tip of the Korean Peninsula, the Haeundae district of Busan draws one million visitors to its beaches every year. Five 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, Man-sik was not able to become involved with Yeon-hee, who runs an unlicensed seafood restaurant, despite her attempts to start a relationship (Yeon-hee does not know that her father was killed due to Man-sik's mistake).
Dong-choon and Seung-hyun's Grandma team up with Seung-hyun and gets involved with some 'illegal' activity to earn money, but the two end up getting caught by the police. Afterwards, Dong-choon's mother finds out that Man-sik's uncle (Eok-jo) was with her when the tsunami hit. She was worried about Dong-choon while Eok-jo was worried about Man-sik. Seung-hyun meets Ji-min and decides to prepare him for a job interview, but Dong-choon refuses. As time passes, Man-sik finally plans to propose to Yeon-hee on a night with 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 named Ji-min (and also a new boyfriend named Hae-chan), they decide not to tell their daughter that he (Hwi) is her real father, afraid that Ji-min would be greatly surprised and shocked.
A wealthy college student from Seoul, Hee-mi, accidentally falls into the sea from a luxurious yacht. Hyeong-sik, Man-sik's younger brother, is a lifeguard who rescues Hee-mi rather 'violently'. Hee-mi becomes greatly angered by the 'violent' rescue and annoys him by following him around. But as time passes on, love starts to grow between them.
When Man-sik is walking to Yeon-hee, he finds Dong-choon. Man-sik becomes very angry. He chases Dong-choon and a bird crashes his car.
Meanwhile, Hwi notices that the East Sea is displaying similar activity to the Indian Ocean at the time of the 2004 Indian Ocean earthquake. The Disaster Prevention Agency assures him that South Korea is at no risk, but a megatsunami soon forms near Japan and starts to travel towards Haeundae. Hwi realizes that the citizens of Haeundae have only 10 minutes left to escape. There is a short earthquake (a little aftershock) at Haeundae; the elevator Yoo-jin is in stops perhaps due to the earthquake. The sea starts receding from the shore, causing mass hysteria on the beaches as people realize that a tsunami is coming. Thousands of people run for their lives, but the tsunami soon reaches Haeundae. Dong-choon, Seung-hyun, his Grandma , and other people on the Gwangan Bridge are swept away by the tsunami. A powerful electric shock from a telephone pole happens on the now-flooded street where Man-sik and Yeon-hee are on, electrocuting everyone in the water and those clinging onto metallic surfaces, but the two survive.
Dong-choon awakens on the bridge to find that a cargo ship has been lodged on one of the bridge's towers, and he tries to escape from the falling cargo containers from the ship. The containers are shown falling at an extreme momentum. Dong-choon survives this, and later on, he tries to light a cigarette while still on the bridge. His portable lighter ends up falling into petrol leaking from a tanker. A huge explosion results that cuts the bridge 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 in which his body is connected to and falls down into the violent sea. The elevator Yoo-jin is in floods with water, and she talks with tears to her daughter on 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 help their daughter get on a crowded rescue helicopter. Before the helicopter leaves, Hwi tells the truth to his daughter that he is really her father. Yoo-jin apologizes with tears to Hwi and they hug each other in front of a second tsunami before they are killed.
After the tsunami, there is a funeral for the hundreds of thousands of lifeguards and people who were killed in the tsunami. Among the ones who were killed is Hyeong-sik. Eok-jo is dead when a box hits him and Man-sik is heart broken. Dong-choon finds out that his mother was killed in the tsunami and he breaks into tears. People are shown helping to reconstruct 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 proposal. The movie ends with the scene of Haeundae in ruins, but in a hopeful atmosphere.
Distribution rights for Haeundae was sold to Hong Kong, India, Indonesia, Japan, Malaysia, the Philippines, Singapore, Thailand, Vietnam, the Czech Republic, Germany, Hungary, the United Kingdom, Australia and Turkey. The film was released in South Korea on 22 July 2009. As of 20 September 2009, Haeundae had received a total of 11,301,649 admissions in South Korean theatres.
In the United Kingdom, the film was released as Tidal Wave on DVD on October 12, 2009 from Entertainment One.