Sam and Dean get a call from Bobby, a hunter friend of theirs, who tells them that he found some clues about their missing father. They meet up with Bobby whom they haven't seen for several years and he tells them about the mysterious death of Henderson, a hunter and friend of their father's when he was in the Marine Corps. Using the clues, the three of them start investigating who the criminal is and they soon discover that it's the same "demon" who killed their loved ones.
