An alien entity, invisible to the living and as large as the Earth's solar system, travels galaxies and stops at many planets/moons to feed by implanting pieces of itself into the unborn living. They then live within those physical lives, all without causing harm or interfering with the lives of the hosts.

At Earth, the pieces have stopped returning to the entity on the death of their hosts, and it's dying without the sustenance they bring. It sends a limited number of special pieces to adult humans around the world. Sent to search for the missing pieces, they can take over the lives of their hosts to do it.

Nate and Liz Crosby are both doctors who were, by luck, both implanted together. Their extensive search on Earth finds none of the missing pieces.

Sam lives across the road. He is struggling through a deep depression after his wife and children are killed. He pretends that he's 'moving on' but plans to sell his house and belongings and travel to a far-off location to stage an accident and kill himself. Nate and Liz learn of his plans. They buy the house and kidnap him before he can leave. Sam holds a piece of the entity, and they want to see if it remains with him in an afterlife.

The doctors kill Sam briefly and he finds a heaven and hell similar to the one described in the Christian religion (even with a beautiful pearly gate) but also very different.

Nate and Liz learn the missing pieces are in the afterlife of Heaven and Hell, and they must find a way to release them.

Little do they know, Heaven will go to war to keep them.