The entire film is in located in a high-rise residential building in Roppongi, Japan where four anonymous men and four anonymous women pay to enter an anonymous sex club. At the beginning of the night the owner comes out and lets them know that they will have from midnight until 5:00 A.M. to continue whatever sexual activities they desire. The rules include showering every time after sex and using the restroom, condoms must be worn at all times, and men must respect the wishes of the women.
When the owner leaves, they are left in silence to continue spending the night as they wish in sexual activity. After the clerk/bartender leaves to get snacks, the men and women slowly begin small talk amongst themselves until the Freeter finally propositions the office lady to go downstairs. From here the remaining men and women slowly paired: the salaried man with the kindergarten teacher, the NEET with the college student, and the factory worker with the regular customer. Despite her timid nature, the college student is revealed to have a powerful hidden sex drive, while the factory worker is revealed to have been a virgin up until this night.
After everyone has had their first round downstairs the men and women open up more to conversation, while the clerk returns with sex toys to be freely used for the rest of the night. As the characters begin to expose their darker secrets, the salary man reveals to be cheating on his wife and the kindergarten teacher reveals to have had sexual fantasies of one of her students. As some of the men and women begin switching partners, the NEET and the college student begin showing a deeper attraction to each other beyond lust. The factory worker, on the other hand, is left to continue pairing up with the regular customer, as neither of them are desired by the other men and women. After more sessions downstairs, the freeter expresses interest in pairing up with the college student based on her energetic passion in bed, but the NEET tries to stop them, to which the freeter asks him if he loves her. Admitting that he does not, the NEET subdues and withdraws, leaving the others to pair up as they choose. However, the scene cuts to show a couple who have joined the others in the living room, creating an awkward silence once again amidst the group. After strong encouragement from her boyfriend, the couple woman propositions and partners up with the NEET, while the couple man pairs up with the college student. Both the college student and the NEET are shown watching each other during their separate intercourse, further implying their growing attraction towards each other. Frustrated with his girlfriend's passion in bed with the NEET, the couple man becomes frustrated with her, explaining how this was a test of their love and she should not have genuinely enjoyed it, and both leave for the rest of the night.
As the night continues, the men and women continue to pair up, with the NEET and college student together once again. After assembling back upstairs, the group appears to be all together again until loud noises are heard downstairs. Re-assembling downstairs, the group applaud upon finding the factory worker and regular customer actively finishing up, with the regular customer proclaiming to have just had the best sex she has ever had, much to the satisfaction of the factory worker.
As 5 A.M. draws the night to a close, the clerk opens the blinds, letting in a blinding white light on the characters. Now under the light, the women shyly put their clothes back on away from the men while the men silently re-dress. When the college student loses her phone, the clerk uses the NEET's phone to call her, accidentally giving him her number. The owner comes downstairs to inform the men that they are required to give the women 15 minutes to leave before they may leave themselves, in order to discourage stalking, and it is revealed that the regular customer and him are in a relationship. After leaving, the NEET receives a text to meet up with the college student, and proceeds to happily meet her at a cafe. The college student, however, asks him to delete her number, claiming that she was not herself that night, and that they cannot see each other again. The film concludes with the college student sitting with her friends at school, while the clerk cleaning up at the club receives a picture message of his newborn child.
