It seems secrets run in their family when they get a call from Bobby informing them of a large secret their father never informed them of...they have a younger sister. A sister named Liz Parker.
In Roswell New Mexico, Liz Parker along with her best friend Maria DeLuca have befriended (and in certain cases more) three aliens, Max Evans, Isabelle Evans and Michael Guerin. Now all 5 are in trouble with the law as they try to keep the FBI from finding out about their secret.
What Liz unfortunately doesn't know is that not only is she the target for the FBI being Max Evans soul-mate, but she's also the target for demons being John Winchester's biological daughter and Dean and Sam Winchester's sister. As for Dean and Sam, once they enter Roswell, they're the target for more than the normal demons, now it's time to battle the FBI.
What will happen? No one knows...but you.