S1, E4: After listening to the story of Na-na and Jung-deun, Ru-mi is certain that Kyu-min is cheating her. In front of Kyu-min, Ru-mi pretends that she does not know anything, but behind his back, she tries to find the proof of the cheating. SNS, apps, messengers... she tries all the digital tools to find the proof of cheating but fails. But one day, she succeeds finding the proof in the blackbox.