what happens in the last 15 minutes?

HAHAHAHA! That would have been more satisfying than the actual ending. What really happened was Donna had a premonition about going in the bathroom, that she would be killed if she did by the killer in the closet. So she wakes up, realizes it was a dream, and like a complete moron, goes into the bathroom <sigh>. The magic killer manages to cut the phone lines, kill the detective in his car outside, kill the cop at the back door, break into the house silently, go upstairs, kill her boyfriend in his bed, and then hide in the closet, all in the time it took for Donna to take a pill. 6 people at the house (uncle, aunt, Donna, boyfriend, 2 cops), all of whom are on high alert knowing there is a mad killer on the loose targeting Donna, no one hears a thing. It was quite an impressive feat.

Then the killer comes out and expects Donna to fall happily in love with him so they can move to Nevernever land, police arrive and dramatically shoot bad guy. The end.