A huge iceberg calves that holds a megalodon (mega shark) and a giant octopus frozen inside it. Deployment of a LFAS (low frequency automatic sonar) unit causes it to shatter. The two behemoths thaw and return to life. The shark terrorizes the California coast while the octopus creates havoc things near Japan. A group of three oceanographers put together a plan to draw the beasts into a trap with pheromones.

Based on a true story about a man named Christopher Gardner. Gardner has invested heavily in a device known as a "bone density scanner". He feels like he has it made selling these devices. However, they do not sell well as they are marginally better than x-ray at a much higher price. As Gardner works to make ends meet, his wife leaves him and he loses his apartment. Forced to live out in the streets with his son, Gardner continues to sell bone density scanners while concurrently taking on an unpaid internship as a stockbroker, with slim chances for advancement to a paid position. Before he can receive pay, he needs to outshine the competition through 6 months of training, and to sell his devices to stay afloat.

Sok-woo, a father with not much time for his daughter, Soo-ahn, are boarding the KTX, a fast train that shall bring them from Seoul to Busan. But during their journey, the apocalypse begins, and most of the earth's population become flesh craving zombies. While the KTX is shooting towards Busan, the passenger's fight for their families and lives against the zombies - and each other.

Two very different couples are drawn into organized crime, but it becomes clear before long that everything and everyone is not what they seem.