A walled city surrounded by lush land, protected by high mountain summits, and fortified by a massive gate to secure the only pass into the valley, The Bastion remained humanity's last hope against the fearsome undead creatures known as the Inferi Scourge. On one fateful day, the valley gate failed and the Inferi Scourge overran and destroyed the human settlements outside the walls, trapping the remaining survivors inside the city. Now, decades later, the last remaining humans are struggling to survive in a dying city of dwindling resources...and dwindling hope.

Vanguard Maria Martinez has lived her entire life within the towering walls of steel. She yearns for a life away from the overcrowded streets, rolling blackouts, and food shortages, but there is no hope for anyone as long as the Inferi Scourge howl outside the high walls. Her only refuge from the daily grind is in the arms of Dwayne Reichardt, an officer in the Bastion Constabulary. Both are highly-decorated veterans of the last disastrous push against the Inferi Scourge. Their secret affair is her only happiness.

Then one day Maria is summoned to meet with a mysterious representative from the Science Warfare Division and is offered the opportunity to finally destroy the Inferi Scourge in the valley and close the gate, reclaiming the lost lands and energizing the populace with renewed hope for the future. The rewards of success are great, but she will have to sacrifice everything, possibly even her life, to accomplish the ultimate goal of securing the future of humanity and saving it from extinction.