After Joe Hardy's fiancée is killed by a hit-and-run driver, the FBI won't let the police arrest the man until their own case against him is finished. Joe takes the law into his own hands and goes undercover as a rich gambler with underworld connections.

Joe Hardy is still haunted by memories of his dead fiancee, but has to focus his mind on the case when Jocco's girlfriend overhears him talking to the police on the phone. Meanwhile, Frank Hardy tries to help Joe by going undercover too, but Jocco finds out and traps Frank in a shark-infested area.

Joe and Frank Hardy go to London to find their missing father, but when Frank goes missing as well, Joe and an agent from the British Ministry must find a traitor together.

In Greece to investigate drug smuggling, the Hardy Boys pose as archeology students on a dig for the lost city of Atlantis, but they are dogged by suspicious accidents and a dead body.

Frank and Joe Hardy journey from the South of France to the Caribbean to find a girl who disappeared while hunting for pirate treasure.

Frank and Joe Hardy are assigned to go to Puerto Rico to protect a senator's daughter from an international blackmailer known as the Scorpion.

Marla falls for an American agent, but Soviet killers pursue her.

A woman executes a securities scam from Atlantic City casinos using a code that the Hardy Boys must break.

Joe and Frank are assigned to protect a motocross racer, the daughter of a mob informant who has been threatened by assassins, but a strange woman has become obsessed with Frank.