After old CLAMS DAGGETT, owner of an excursion launch, nearly runs down FRANK and JOE HARDY and their friends, the youths decide to buy a boat and compete with Clams in ferrying passengers to Rocky Isle. Most of them have money to contribute, but JIM FOY has an uncle who has a Chinese junk for sale. The Hardy boys pile into their car with TINY, a Great Dane, promising to call their dad and get permission for the venture. When they get home, AUNT GERTRUDE is at the door with Dr. MONTROSE, a newcomer, whom Tiny pounces on lovingly until the boys pull him off...

In the Curator's office of the Los Angeles County Art Museum, several men are discussing 'the progress made by FENTON HARDY, the boys' private investigator father, when Hardy himself enters. He has evidence proving that the stolen jade they hired him to find has been smuggled into the country. He is interrupted by a call from the boys on the shortwave radio he carries in his briefcase. Giving an okay on the boat purchase, he tells them and the men present that his prime suspect in the jade case is a man called Ballarat, an expert on rare jade...

On a San Francisco dock, the boys close the deal for the junk; but before they can sign the papers, BURKE and two henchmen, a tough trio, come to buy the boat. When they're refused, they move in menacingly and Jim employs Karate to scare them off. At a Chinese dinner that night, Frank gets warning in a fortune cookie against their voyage home. Deciding to leave that night, the six boys fight off a gang of toughs in an alley and then discover an interloper, GEORGE TI-MING, aboard the junk. He also wants to buy the junk; but when the boys say they won't sell, he leaves...

As the junk nears home, a bogus Coast Guard boat pulls alongside, but the boys, suspicious, repel the boarders, who turn out to be Burke and his men...

When Frank and Joe return home, Tiny is barking excitedly, driving Aunt Gertrude mad. The explanation is an electronic signaler in his collar set to receive signals from Frank's belt buckle. Their father has asked for the file on Ballarat, but it's missing from the files...

On their first trip to the island, the group discovers that someone has tried to scuttle the junk, but Clams, who tells them to keep a guard on board, is not the guilty party. Using the briefcase transmitter, the boys reach Hardy, on stakeout at the waterfront, and bring him up to date. As Hardy signs off, two men enter his room and knock him unconscious...

Meanwhile, CHET, the boy guarding the junk, accidentally finds a bit of jade hidden in a compartment. Frank and Joe try to snoop around Montrose's mansion, but Montrose, who is actually Hardy's original suspect, Ballarat, welcomes them and they find nothing until Frank is attacked and stuffed in a laundry chute where he finds another bit of jade...

When the group gathers later, they find Chet' s compartment empty, but Frank notices a small particle which he and Joe take home to analyze and find it's from Rocky Isle. Ti-Ming is one of the passengers disembarking on the island along with Frank and BIFF who are going to investigate the caves while the others, with Tiny, run the junk back and forth. The two boys enter a cave where they are surprised by Burke, who already has Hardy under guard. Ti-Ming also a private investigator, gets the drop on Burke, but another man jumps him and he is tied up with the others. Montrose arrives, and the four watch him buy the stolen jade from Burke. Frank, activating his buckle signal, saws through his ropes and frees the others while the boys from the junk race to the cave followed by Tiny who, upon hearing Montrose’s voice, leaps happily on him slathering him with kisses...

The others overpower Montrose, Burke and his mob and put them aboard the junk, where Tiny alternately growls at Burke and kisses Montrose all the way to the mainland.