Marcus was invited by his girlfriend to a camping trip. Her intention was to break up with him so she could pursue her studies in Washington, and she doubted she could stay with Marcus at the same time.
Marcus was oblivious to this, and while Gwen was preparing to say goodbye to him, Marcus wanted to marry her. In the woods, Marcus went to collect more firewood, leaving Gwen with his belongings. She found an engagement ring inside his bag, but this did not deter her from her goal, as she did not truly love him, despite caring for him.
Gwen was distracted when she heard Marcus scream. He was running back to the camp, and urging her to run too. His leg was injured and just then, something invisible knocked him to the ground and began clawing into his back. Marcus died immediately from the wounds, and the creature proceeded to go after Gwen.
She was able to fight it off using an axe. When her story reached Sam and Dean, they paid her a visit in the guise of FBI agents. They confessed that Marcus had been killed by a hellhound, a monstrous dog straight from Hell. They initially assumed Marcus had made a crossroad deal, but Gwen believed this was unlikely.
Sam and Dean decided to hunt down the hellhound, and Sam was successful in killing it. Gwen was left feeling somewhat guilty since Marcus had died right before he was about to propose to her, and right before she was going to break up with him. Gwen blamed herself for his fate.