In either a near-future or alternate present reality, the Earth lies nearly desolate after extensive overuse of its resources in order to colonise other planets.
As a result, the remaining inhabitants that have not emigrated are forced to live on the drained ocean floors, and have become nocturnal so as to avoid the scorching heat and radiation of the Sun during the day.
The last orbiting launch pads are falling out of the sky to the salt dunes and corals towers that were once the ocean bed, and after the last have fallen, the Earth is truly abandoned.
One day Holliday and Granger are in a saloon fittingly called the Neptune, when they see the crash of one of the decadent launch pads falling from the sky.
When Granger asks of his fascination, Holliday explains that the shark is like an image of themselves staying behind and equates the situation with primordial evolution.