With a dystopian ambience, "Billennium" explores themes similar to Ballard's earlier story "The Concentration City", of space shortages and over-crowding.
The story is set in the future (possibly c. 21st century – see billennium) where the world is becoming increasingly overpopulated, with a population of around 20 billion.
In the city inhabited by the two protagonists, John Ward and Henry Rossiter, there is a mass shortage of space and the people live in small cellular rooms where they are charged by ceiling space, the legal maximum decreasing to 3.5 square metres (38 sq ft) per person.
The city streets are enormously crowded, resulting in occasional pedestrian congestions that last days at a time.
The story revolves around Ward and Rossiter's combined discovery of a secret, larger-than-average room adjacent to their rented cubicle.