Obsessed (2009): Party in the men's room

What’s happening in this Obsessed movie clip?

Derek is in the men's room and when he walks into the stall, Lisa suddenly appears and follows him inside. She is all over him, trying to kiss him, pulling his shirt out of his pants as he tries in vain to avoid her. They hear a drunk guy at the urinal who's trying to peek into the stall to see who's inside making out. He keeps shushing Lisa and she keeps trying to get into his pants. He forcefully pushes her away, tells her to stop and leaves the mens room. He grabs his jacket and hurriedly leaves the party.