In Hong Kong, Kelly and Scott are visiting their host, a mandarin, unaware that a sniper has set up across the street on a hill. He draws a bead on Kelly as he finishes the game... and fires just as Kelly's opponent Courtney James shakes hands with him at the end of the match. Afterward, Scott gives Kelly a massage while Hong Kong policemen Hardy and Maitland question Kelly. He explains that he's in Hong Kong to play tennis and travels the world living with wealthy patrons, with Scott as his trainer and masseur. The officers assume that the sniper was going after Kelly, who claims he has no idea why anyone would want him dead. They tell him that if he tries to leave Hong Kong in the next two weeks, they'll put a hold on his passport.
Once the officers leave, the partners try to work out who would be after them when they're not working a case. Kelly figures that their cover is busted and the killer will try again. As Kelly checks his luggage, Scott attacks Kelly, who thwarts the attack and points out that he's two ahead. As they talk, Scott notices someone opening the door. A woman comes in and tells them that there's a party in the garden and Kelly is wanted.
As Kelly flirts with a girl, Scott pulls him away into the garden and points out a man seated nearby. The agent has spotted a gun beneath his jacket and a throwing knife strapped to his calf. They go over to say hello to the man, Edwin Wade, draw a gun on him, and lead him away. When they check his ID, it shows that he's a master tailor out of New York. He has their orders, which tell them to check out the Chinese government's interest in a missing train. Wade takes his weapons back and explains that a freight train with six boxcars left Canton bound for Hong Kong. It crossed the border into the new territories and then disappeared. It was carrying stone, sand, and cement and one of the Pentagon men got killed asking questions. Scott tells Wade about the assassination attempt but he figures that it's part for the course. He tells them to meet with Wong Lee, a freelancer who worked with them in Saigon. He knows something about the train but won't talk to anyone but the agents. Wan Lee is waiting at the Tiger Balm Garden, which is closed because a typhoon is brewing.
The agents arrive at the gardens and Wong apparently calls them up from the level above. As they go up the stairs, a statue next to him falls off the edge. The agents narrowly avoid it and run after the man. When they get there they discover the man is dead, a knife thrust into his chest.
Back at the manor, Kelly is clearly upset at the death of his old friend. Scott assures him that all they could do was make an anonymous call to the police, but Kelly vows to find the train and the man who killed their friend. They board a train heading toward China and the killer, Gavin, sneaks up behind them and aims his silenced gun. A passenger keeps getting in the way, blocking his shot, and the passenger finally comes over and offers to exchange American cigarettes for his local ones. Kelly gives him the pack and the passenger says that he saw Kelly play. He explains that he's a tennis player as well and suggests that they exchange autographs. Kelly isn't interested but soon starts talking about his experiences. As the train approaches Tai Po, the last town before the frontier, Gavin decides to wait until later.
The agents get off at Tai Po and start walking the tracks north. As they come to the middle of nowhere, they hear a train bell in the distance. The agents spot a nearby walled village and go to investigate. They talk to the villagers of T'Sien Cha and ask about the bell, but no one seems to know what they mean. Kelly notices an American woman standing nearby, wearing Chinese garb, and goes over to talk to her. She introduces herself as Rachel Nelson and explains that she teaches school there. Kelly says that he's just looking around and Rachel wishes him well. As Kelly and Scott enter the fort, Gavin watches from a distance.
No one knows anything about a bell and the agents keep looking around. Scott sees someone in an alley point a gun on them and they sneak up to the corner, only to discover it's a small boy with a toy gun. The boy, Lee Ho, tries to rub off Scott's black skin coloring and explains that he learned English in the local school. Scott asks him about the bell but the boy emphatically says that there isn't one there and runs off. As the agents continue on, a woman pulls them over and takes them to a small home. An older man introduces himself as Yau Shui and explains that he is the equivalent of mayor. Yau Shui asks why they are inquiring about a bell and they explain about the missing train. The mayor offers a toast to Mickey Mantle and Sandy Koufax and then suggests that they leave. Undeterred, Kelly warns him that they're not going anywhere and Scott points out that Yau Shui's wife is using a locomotive oil can to water her flowers. Yau Shui asks why they're interested in the train and asks if their lives are important.
Scott and Kelly leave and Lee runs past, looking for firecrackers for the festival. Gavin finds him and trades his toy gun for a real one. Meanwhile, the agents consult a fortune teller, who tells them that if they stay they die. Rachel is standing nearby and Kelly goes over to talk to her. She explains that the villagers are celebrating the Feast of the Excited Insects and they go for a walk into the countryside. As Kelly takes her hand, Rachel asks why he's there, well aware that he's a tennis player. He avoids the question and says that he'll soon have to leave. As they come back to the village, Rachel asks Kelly to leave T'Sien Cha as a favor to her, insisting there is no bell there. The train bell rings nearby and Kelly runs off to investigate it over her objections. Kelly finds Lee Ho in at a construction site, ringing the bell.
In T'Sien Cha, Scott hears the bell, goes to the mayor's office, and tells him that they have a lot to talk to.
Kelly thanks Lee for ringing the bell and climbs up to check it. As Rachel tries to explain, Lee keeps telling Kelly that he wants to play gunfight but the agent ignores him. Rachel tells Kelly that the villagers needed the train, and Kelly figures that they heisted the train. She didn't know they were doing it, but they did it for her. The engineer and the fireman were villagers and diverted the train onto an old spur, and then buried it. Rachel realizes that Kelly is more than he seems and kisses him, but he still wants to know why two men are dead. The agent figures that there was something else on the train.
Before Rachel can say more, Lee opens fire on the two of them. Kelly gets Rachel behind some cover, but she insists on approaching the boy. He finally hands it over and Kelly fires a shot at the bell to demonstrate that it's a real gun. Lee breaks into tears and Kelly hugs him. The boy tells him that a man gave him the real gun and told him to ring the bullet, and Kelly sends him home. When he points out how dangerous the people looking for the bell are, Rachel agrees to show him.
Rachel takes Kelly to the nearby harbor of Aberdeen and they take a boat out to a sampan. An old man, Lou Wai, and Rachel explains that he's a Chinese nuclear scientist who was smuggled out on the train. Lou Wai isn't interested in politics and just wants to stay in Hong Kong. Rachel asks what Kelly will do with him. Before he can say, Gavin comes in and thanks Kelly for doing all of the work for him. Now he plans to dispose of Kelly, Lou Wai, and Rachel. Kelly knocks over a lamp, distracting Gavin long enough to get the drop on him. The two men struggle and Kelly finally defeats his opponent. Rachel runs over and kisses him, and Kelly takes Gavin to shore.
Later, Kelly writes a note to Rachel explaining that he's taking Lee to Hong Kong on a spree. Lee gives the note to Rachel, who smiles. Scott has joined the local festival in traditional garb and tells Kelly that they made him a living legend because of his skin color.