Tom Swift's father has been working diligently on a secret project, which he reveals at the beginning of the book as a submarine.

While in New Jersey to launch the submarine, Tom reads in a newspaper that a ship named the Boldero sank off the coast of Uruguay during a storm, taking down with it the sum of $300,000 in gold bullion.

Tom persuades his father to pursue this treasure as opposed to competing for the government prize.

The other submarine, named the Wonder, soon sets off to follow Tom and his crew after they embark on their journey.

Tom knows that the Wonder and its crew is not certain of the location of the wreck, and is merely following the Advance, hoping to steal the treasure at the last moment.

After the Wonder tries to ram the Advance, Tom and his father take to the heavy underwater cannons, and successfully disable the Wonder, leaving her damaged and immobile.

An engine mishap forces the Advance to surface off the coast of Brazil, where they are soon confronted by the Brazilian battleship São Paulo.

Tom and his crew are captured and scheduled to be executed two days later, and the submarine turned over to the Brazilian government.

The night before their execution, a hurricane strikes, and the São Paulo is pushed aground by the winds.

In their extreme-depth diving suits, Tom and Captain West enter the waters where the wreck is, which is at a depth of over 2 miles—similar to the RMS Titanic.

Gold was found in a secret compartment behind the Captain's safe, and recovered from the Boldero just in time to escape from the now-arriving Wonder.

She is short of stature and has to stand on a soap box to kiss Tom goodbye on one of his voyages.

Tall, thin and 'dark' of complexion, he seems to now have given up speaking in short, choppy sentences, as in the 1st three books.

"In residence" with the Swift family, has become sort of hands and feet for Tom's father.

Andy Foger Red haired, squinty-eyed bully, who makes great trouble for Tom.

"Poor little rich kid", son of wealthy family, born with a chip on his shoulder.

Lately has upped ante to attempted murder when he traps Tom in an airtight compartment and leaves him to smother.

Ned Newton Chum and companion of Tom, currently employed in Shopton National Bank.

Mr. Wakefield Damon Elderly and eccentric adventurer whose main purpose in life seems to be blessing everybody and everything near his person.

In spite of ongoing trouble mastering most any conveyance he tries to control, has successfully piloted an airship.