In present-day Manila, former high school classmates, Damian and Nanachi, re-establish communication after eight years. They catch up on each other only for Damian to find out that Nanachi is greatly depressed by her recent heartbreak with her ex-boyfriend. Damian suggests an experiment to Nanachi and explains that when you do something for 21 consecutive days, it will become a habit. He proposes to Nanachi that if she ignores her ex and distracts herself from the heartache, she'll find it easier to move on. In the process, they both reminisce that back in high school, they could have actually ended up with each other if only Damian showed his motive to Nanachi. The 21 Days Theory is a short story about youthful love, future regrets, and realizations.