In THE 12TH PLANET, Zecharia Sitchin argues that Sumerian myths of a pantheon of gods are no less than fact and that Earth's solar system includes an additional planet called Shar which comes near Earth every 3,600 years on an elliptical orbit that takes it deep into space. The planet, he claims, is inhabited by advanced alien beings called Nefilim. Based on a Sumerian origin myth, he recounts how the arrival of this planet formed the solar system as we know it, and he credits the Nefilim for advancing humanity and also for the stories of the Great Flood.

Sitchin gives a history of the archeological investigation into ancient societies, tracing the known culture of ancient Greece back to earlier cultures, finally arriving at the earliest known civilization of Sumer. He gives the impression that earlier cultures were more advanced than later ones and that Sumer arose suddenly and was amazingly advanced. In this way, he shrouds Sumer in mystery. Similarly, Sitchin gives the stories of the Greek and Roman gods and traces similar stories through other cultures, arriving at Sumerian mythology.

The main mythological characters are Anu, a distant father god, and two sons, Enlil and Enki, who vie for superiority and power. The gods come from above and travel to Earth, walking among the people. Sitchin interprets the Sumerian myths as being factual accounts of alien beings from an extra planet in Earth's solar system that came to Earth 450,000 years ago to mine for gold and other rare minerals. The aliens had helicopter-type flying devices to move around Earth, powerful weapons, and rockets to travel to their planet.

Sitchin claims that the missing planet, Shar, follows a 3,600-mile elliptical orbit from deep space to the asteroid belt. Coming from outer space, the planet crashed through the solar system, pushing Pluto onto its current orbit and cutting one planet in half, creating Earth out of one side and the asteroid belt out of the debris. It also knocked the moon into place. Sitchin gets his claims from his interpretation of Sumerian myths as factual accounts.

While on Earth, the inhabitants of Shar, which Sitchin calls Nefilim, genetically engineered humans as a slave race to help with mining. Enlil became unhappy because Enki gave humans the ability to reproduce (a reinterpretation of the story of the Garden of Eden). Enlil decided to destroy humans, and so did not tell them of the coming Deluge (the story of Noah.) Sitchin interprets the Deluge as tidal waves from the end of the last ice age. Enki played the role of God in saving "Noah." Mankind survived, and after the Great Flood, the gods gave mankind agriculture and helped humans build civilization. The humans tried to build a rocket ship, though (a reinterpretation of the Tower of Babel), and the gods broke them into three different groups, giving each a different language.