Thanks to a mutating virus that all but wiped out humanity, life is short and brutal out on the Platform.
The dilithium crystals that power your respirator are the very same thing that will kill you when their runtime is up. Unless you can survive your Interval and be among the chosen few lucky enough to travel on to the paradise known as Tranquility Base.
That's not a choice for Kitteridge, though; he's just witnessed the brutal murder of his wife and the kidnapping of his newborn child by the drones of the Collective. His only choice is to somehow survive his own Interval and hope he can dodge the Waster Kings' gangs long enough to find his child before the dilithium poisoning eventually kills him.