"Man's Best Friend" stars Louis Koo, Jessica Hsuan, and Sammi Cheng. Just by looking at the list of actors and actresses in this series. Louis Koo is a policeman who works with a dog name Rocky. Amazingly, Rocky understands human emotions and is quite human. One day Sammi's purse gets stolen and Louis helps her get it back. Louis falls in love at the first sight of Sammi. However, Sammi already has a boyfriend. Jessica Hsuan is an assistant vet who loves animals and owns a dog named Dong Dong. Louis happens to win the lottery but Dong Dong snatches the ticket. When Louis tries to retrieve it, Jessica mistakens that Louis is trying to hurt her dog. She and Louis get on the wrong foot. The next day, Jessica discovers that Dong Dong is gone and believes that Louis took her dog. Louis happens to find a dog and gives it to Jessica to adopt. Meanwhile, Louis covers up for an affair that Sammi's boyfriend is having but latter Sammi finds out and the two break up. Louis moves in with Sammi because she couldn't sell her apartment.
They eventually become close and Louis tells Sammi his feelings. They start dating but as Sammi pursues her career, their relation slowly drifts apart. Meanwhile, Rocky and Jessica's dog elope and Jessica is very mad because she doesn't like Louis. At the police station, Louis gets a chance to be promoted but he needs to be interviewed in English. Louis asks Jessica to tutor him but he misses the interview because of Sammi. As Sammi's career progresses, Ken decides to send Sammi overseas. Louis realizes that Sammi's career is more important than their relationship. They break up and Sammi moves out. Louis has a new dog, Bobby. While they were on patrol, Louis stops an armed robbery and everyone congratulates him as a hero.
Jessica tells Louis to talk to Sammi and Louis finds out that he has no feelings for Sammi anymore. Sammi gives Louis a plate for a present and Jessica also bought the same plate for Louis. Jessica falls in love with Louis because they spend so much time together but doesn't tell him. A man named Hoo falls for Jessica but Jessica pushes him away. Hoo keeps on sending Jessica gifts and calling her constantly. One day Jessica tells Hoo over the phone that she is in love with a policeman, but the guy on the line was Louis. Rocky decides to step in and help Louis and leads Louis to the plate that Jessica bought him.
Meanwhile, Jessica uses all her money to save the pet shelter and Louis is touched. One day a storm hits, Louis' and Sammi's boat drifts away but Louis is stuck at the police station. Jessica saves the boat and gets into the hospital. Louis comes and visits her but Jessica pushes him away, leaving Louis very confused. Jessica runs a little short on money and can't make rent. Louis sells his boat to pay for Jessica's rent and so Jessica and Louis are in love. I'll stop the story here, about halfway through the series. Watch the series to know the ending and how the entire plot unravels.