In a series of hair-raising adventures both on land and undersea the teenage brother detectives pit their wits against some of the most ruthless criminals they have ever encountered. It all starts when Frank and Joe are skin diving just for fun and the thrill of exploring the undersea world. Suddenly, deep in the waters that flow near the foot of Pirates’ Hill, a mysterious skin diver fires a spear through Frank’s air hose. From this moment on, danger is never far away. The very lives of the boys are at stake as they, along with the help of their pals, uncover a mystery involving an old Spanish cannon and a fabulous sunken treasure.

Hired to locate an old Spanish cannon, the Hardy brothers uncover an even greater treasure after perilous underwater adventures.

A hidden door. A magical staircase. Discover the world of Droon! Everything in Droon is changing. Lord Sparr has disappeared. Ko and Gethwing have fled to the Dark Lands. And an old villain is back -- Ving, leader of the Hawk Bandits! This time, he has his twin sister in tow, along with her band of wicked pirates. Arrrg! They're snatching magical stones from all over Droon, and using them to build an enchanted passageway that will transport nasty dragons from the past into the present. Can Eric and his friends put a stop to the pirates? Or will they be lost at sea?

Nancy meets Helene and Henri Fontaine, refugees from Centorvia who run a dancing school in River Heights. Strange circumstances have brought the brother and sister to the United States. When they receive an anonymous note threatening their lives, Nancy offers her help, but she encounters nothing but puzzles. Are the Fontaines involved with the Centrovian underground? Have they been threatened by their own countrymen? When the Fontaines disappear, Nancy and her friends pursue the trail, even though danger lurks around every corner.

When she is told that a large inheritance from a Greek tycoon, meant for her friend Helen Nicholas, was stolen, Nancy agrees to find the culprit. A poisonous snake in a basket of apples and a strange symbol stamped on a rare Byzantine mask are clues in this mystery set in the beautiful and exotic country of Greece. These clues lead Nancy and her friends to a ring of art smugglers and to the secret of the Greek symbol.

Caught in their motorboat the Sleuth by a sudden storm at sea, Frank and Joe Hardy are relieved to see the lights of a yacht. But their SOS ignored. The shrill sound of a siren fills the night air and moments later a helicopter drops an object alongside the Sleuth: a wallet containing two thousand dollars! The next day, a truck carrying a shipment of high-powered rifles has been stolen. These two apparently unconnected events are clues in a tangle of mystery which turns out to be one of the detective brothers’ most exciting adventures.