Sam doesn't listen to advice and keeps jumping back to the past, without Jenna's help, to try and catch the serial killer he may have created. He waits in the cupboard of the third victim, Anita Barnes (who shares his love for whiskey), and watches as she is attacked -- but it turns out to be her boyfriend, as she is into some sick rape fantasy thing. He is found hiding, and sent forward, confused. He finds out soon enough that Anita Barnes is still alive, but has taken out a restraining order on him, and the third victim is now Lonnie. Goldberg has also disappeared - and it looks like it was 'future Sam the murderer' as Jenna reveals he was about to go public.
Sam goes for a drink at the bar and is turned down by Vicki in this timeline. As he leaves, Vicki is brutally murdered by the masked chainsaw murderer. Sam's name was on the last receipt from the bar so he is hauled to the station again, and released later by Jenna and a high-price lawyer. He has stolen a police diary of the events, and uses it to jump needlessly to the location before the murders were committed. It's not clear whether he sees anything other than "Welcome Home" written in blood on the walls.
Back in the future, Jenna is a different person to her character at the beginning and growing in confidence. Sam returns to the crime scene, again for some weird reason, where the police pick him up. In the cells he uses his "joker" card to call Detective Glen, and reveals that he isn't a psychic but a time traveller - and "proves" it with a weak bit of info that his wife thought he looked like MC Hammer. Glen risks his job to release him.
Sam goes home, and smells toxic flowers on Jenna's bed, starting the paralysis in motion. He drags his limp body and some ice to the bath and manages to jump just in time to the crime scene again. He finds Goldberg, badly beaten in the bloody room, and runs to get help - but is snared by a hunters tap.
The hooded chainsaw serial killer approaches him ... and removes the hood ...
And it is Jenna!
She cares for her brother and reveals the twist - she can jump too. She has told him many times, in other timelines. She loves him, a little too much, and is jealous of the other women in his life - the other women murdered were his future/past girlfriends, and Lonnie who cheated on him with Rebecca. She tries to make a move on him. Sam quite rightly is repulsed. Without ice or anything else, he time travels to the fire when he was 15.
In another disturbing scene his parents escape, but he keeps the door locked on his sister so she is burned alive. He leaves the house, and his parents break down when they see that he didn't rescue his little sister.
Back in the future, Sam is now married to Rebecca's sister, and they are travelling to a family barbecue - with their daughter, called Jenna, in the back. At the BBQ are his parents and friends, including a fit again Goldberg. His daughter walks up to the 'barbie' and places her Barbie on the flames and smiles with a psychotic grin. I thought this meant that she is somehow the reincarnation of Jenna, or Jenna somehow jumped into her body or something, but it might just be a coincidence thing that the serial killer gene runs in the family and they'll have trouble when she hits those teenage years!