A sudden accident shatters the everyday happiness two couples had always taken for granted — and exposes a secret one of them kept hidden for years.

Itsuki reveals he saw Azusa and Mitsuru together on "the third Friday." Sakiko wants to believe her husband, but begins investigating their secret.

As their investigation stalls, Sakiko and Itsuki bond over shared frustration. A call from the police leads them to retrace Mitsuru and Azusa's steps.