Park Eun Hyung's debut film Heart Is... stars Yoo as Chan Yi, an 11-year-old boy living alone with his 6-year-old sister, So Yi. On the day of her birthday, he steals a little puppy to give to her as a present, a present that will become the most important companion of their young lives. Named Heart, the dog keeps growing and helping the two children. When a terrible accident occurs, however, Chan Yi's relationship with Heart changes forever.
