网络设计师一子 （ 高畑充希 饰）和在文具制造厂工作的丈夫二也 （ 冈田将生 饰）是像朋友一样什么都能说的“好朋友”式夫妇。但婚后无子女的二人有一个秘密。那就是，每个月的第三个星期四晚上，二也会和恋人一起度过。因为为了维持夫妻关系，两人选择了“婚外恋爱许可制” 。

Ichiko and Otoya are happily married, despite not having sex for the last two years. The secret to their happiness is Otoya's affair with Mitsuki, which Ichiko approves. Everything is going well until Otoya rejects Ichiko's advances on the night of their seventh anniversary. In shock, Ichiko seeks advice from her friend who suggests she exercise her right to have a boyfriend of her own.