留學韓國的中國學生趙明住在朋友東宇家，他家的狗媽媽心心生了三隻可愛的小狗，其中一隻叫小心。某天，小心不幸落入搶劫者之手，狗媽媽心心力克艱險從其手中救回小心，並幫助趙明、東宇抓獲了在青島搶劫珠寶的兩名搶劫者。

Dong-wook, in his late teens, is falling behind in his studies and has already transferred schools three times. Dong-wook’s only friend is a Labrador named Hearty, a present from his late father. When Hearty gives birth to three puppies, Chubby, Dodo and General, Dong-wook becomes even more obsessed with Hearty and raising up the puppies. Worried and displeased, Dong-wook’s mom decides to send Hearty and the puppies away to her brother’s place without giving them a proper goodbye.

A major diamond theft makes the national headlines. Hiding from the police, the burglars conspire to siphon off the diamonds abroad by concealing them in a stuffed animal, so they kidnap General, the new puppy in town. Witnessing General’s kidnap right in front of her eyes, Hearty chases after the burglars’ truck like a bullet. When Dong-wook arrives at his uncle’s place to take back Hearty and the puppies, he finds that Hearty and General have gone missing. While Dong-wook is searching desperately for them, Hearty locates the burglars’ hideout in an abandoned school. Hearty rescues General, swallows up the diamonds and runs away, and so begins the chase between the burglar and Hearty, a dog that can outsmart them all.