When his girlfriend gets stuck at work and misses the last train, he is left alone with her adult sister Ai. She hints that her sister may be cheating, flashes her F-cup tits, kisses him, and offers a handjob, blowjob, and titjob. He tries to convince himself it is not cheating as long as there is no penetration, but Ai claims her sister really is unfaithful and offers to take her place. Confused, frustrated, and overwhelmed by her temptation, he finally gives in and has sex with her.