Season 11 Edit
Dan and Melissa have been married since they were young, but at some point, Dan began secretly dating their babysitter, Staci Altman, who is much younger than Melissa.
Staci urges Dan to divorce Melissa quickly, but Dan declines because it happens to be Valentine's Day. He leaves Staci at the house while he and Melissa go out for the night.
He and his wife return to find Staci dead. The next morning, Sam and Dean arrive along with the forensic cleaning service. Dan tells them that whoever met Staci, "falls in love with her". He also spends considerable time watching Staci's body get packed up.
At his office, Dan looks at Staci's Facebook page. He logs off when Dean, still pretending to be FBI, comes in. Dean manages to convince Dan to be honest about his affair. Dan reveals a "nanny cam" he took from the house, since Staci had kissed him in front of it. Dan swears he was going to hand it over to the police, but couldn't.
Dan reveals the video showing "him" killing Staci and walking away with her heart. He tells Dean it wasn't him, saying he was having dinner all night among over 75 people. Dean tells Dan that he believes his innocence before taking the video with him.
Dan is later confronted by "Staci". He is horrified to see her and shouts that she should be dead. "Staci" ignores his words and proceeds to punch through his chest, killing him.