You shouldn't sleep with your partner's best friend.
Mark is hanging out at his friend's James house when his friend's girlfriend Christina flirts with him.
Mark doesn't want to hurt Christina.
Mark responds to her behavior with a compliment and funny story about his friend.
Mark goes along with Christina flirty behavior, and they have an erotic affair.
Christina thanks Mark for reminding her of the reason she loves James.
Mark and Christina both lose their relationship with James.