At the end of season seven episode seventeen, Castiel had been able to rebuild the wall in Sam's mind. So instead of Castiel having to stay behind and be tormented by Hallucifer, he left with Sam, Dean, and Meg. The four soon met up with Garth, and due to Castiel saying they were needed in Sioux Falls, that's where the crew headed toward.
Sioux Falls has become a demon hot spot. Not only are the demons taking over but there are plenty of angry spirits on the loose,a and more then enough fallen blood. The hunters that have already gone there are having more then enough trouble keeping the town in tact. But it isn't the horrors in Sioux Falls that await the brothers, but a little something their father failed to inform them. A surprise in the form of a twenty-three year old sister named Emma; the outcome of a one night stand. She just happens to also be a hunter.
Aside from the sister and horrors in Sioux Falls, there is something else brewing. Something dark and evil is waiting for just the right moment to strike and with it, it hopes to destroy the balance of good and evil.