Brink takes place on the Ark, a floating city built to demonstrate and develop sustainable construction and living technologies. It was only designed to support a population of a few thousand Founder investors, research scientists, engineers, the guests and staff of a luxury eco-hotel and a small security detachment. The problem was, the seas began to rise much faster than anyone had imagined. The Ark was moved to a secret mid-ocean location, but even so it was found by tens of thousands of desperate refugees and some millionaires who'd given up the last of their now useless riches for the promise of a one way ticket to safety. These refugees, ironically referred to as "Guests", are reduced to living in rusting floating slums. And then, suddenly, the ships and planes stop arriving, and the Ark is completely cut off from the outside world.
Fast forward to 2045, and the Ark is wearing out. There's a fragile balance of power - no one faction predominates, although the Founders have a much easier life than the Guests. The Guests do all the work, but are dependent on the Founders for employment, power, heat, light and food. The Founders feel they have done enough to give the Guests a home and any requests for additional resources should be earned. However, the Founders depend on the Guests to keep the Ark running - the thousand menial jobs that keep the Ark alive all have to be performed by Guests. Captain Mokoena's Security faction protects the status quo.
A Resistance movement has formed, led by a charismatic union leader named Joe Chen. The Resistance's aims are various and somewhat incoherent. Some want to overturn the power structure entirely, some merely to gain better political representation and more equal distribution of the Ark's resources. Some are insistent that the priority must be to re-establish contact with the outside world, reasoning that the there's no way the seas rose high enough to destroy all human life. Brother Chen has managed to keep the various strands of dissent united under his leadership.
As long as the Ark remains self-sufficient, the current status quo can endure. However, rumours are beginning to spread that the Ark is running low on key resources; that it no longer has the spare parts to maintain or repair key facilities, that the water desalination plants are failing, and that soon there will have to be a violent struggle for control of the Ark.