"I was suddenly presented with a marriage certificate by my childhood friend, whom I had jokingly promised while drinking that we would marry each other if we were both still single when we turned thirty."

Mizukusa Hideo and Shakut束Ito are childhood friends. Born on the same day as neighbors next to each other—a relationship forged in steel. Then, at the moment Ito turns thirty, she thrusts a marriage certificate at Hideo. The trigger for this is something he said five years ago on a whim while drinking: "If we're both still single by thirty, let's get married." What will become of this marriage certificate?