Alien Citadel is the final book in a trilogy about Finn Ferral. In the first book, The Huntsman, Finn's quiet life shatters when spywings kidnap Josh and Jena and enslave them. Finn sets out to rescue his father and sister, and his search leads him into many strange and dangerous situations, including encounters with unexpected information about his own origins. In the second book, Warriors of the Wasteland, Finn and his Bloodkin ally Baer join with warrior bands to battle Claw, a human collaborator with the Slavers.

As Alien Citadel opens, Finn has found his sister and defeated Claw, but these victories have only led to a greater threat as the Slavers decide to rid the Earth of its troublesome "wild human vermin."