To save you watching the whole thing (spoilers)

I can't be bothered writing a full synopsis, as it's right down there with the worst films I've seen in the last few years (even the worst made-for-TV movies), so if you've got to about the half hour mark and you're ready to give up but you still wanna know whodunnit...

They make out like it's the blonde girl, Diana, who did it. Her and Paul were boyfriend / girlfriend when they were kids, he gave her some crappy fabric ring, and she's been in love with him since then. She has a shrine to him which Jessica and Josh find.

Then she goes psycho and attacks Josh, and attacks Jessica, and goes to Paul's house and attacks Paul, and sets his house on fire, leaving him for dead, but he's okay. Big show down between Jessica and Diane with underwhelming fight scene, Jessica triumphs, and Josh is gonna be okay too, huzzah.

Then we see Karen's funeral and Paul playing grieving husband. Then he goes and visits Diana in jail and we find out they were in it together - he convinces Diana he would wait 25 years until she was out. Then he runs off into the sunset with his mistress. Fade to credits.

Hopefully I've saved many people the hour of my life that I lost there. Any questions? Don't bother. Put them out of your mind. This isn't worth losing any sleep over.