The N'Am-Pac Grid Authority is building geothemeral power plants at the bottom of the ocean, along tectonic rifts. But constructing and staffing these stations is a problem, since people are hard to keep alive and functional at such depths. As such, the company has developed certain cybernetic enhancements for its workers, resulting in the titular "Rifters". With one of their lungs replaced with complex machinery, a Rifter can breathe underwater indefinitely as well as function in the crushing depths of the deep sea. Combined with a self-cleaning skinsuit, a voice-modulator for speaking underwater, and light-amplifying "eyecaps", a Rifter can feel more at home in the deep than anywhere else.
The thing is: normal people tend to suffer complete nervous breakdowns when they're miles underwater, literally under extreme pressure where they might die at any moment from the many dangers of the rift. So in a somewhat questionable bit of decision-making, the company decides to recruit new Rifters from people who are already psychologically damaged - in other words: abuse victims, war vets, criminals, etc. This ends up working surprisingly well since these people, already used to living in constant stress, are better able to mentally handle the rift. Of course, this can (and does) lead to a bit of friction between the staff, seeing as they're all disturbed individuals. This becomes a recipe for disaster when, deep within the Juan de Fuca rift, an ancient microbe is discovered. One that has been isolated for over two billion years and that could mean the end of all life on the surface. And the people put in charge of containing it feel less and less connected to humanity every day...