Barton & Tom are trying to complete Barton's prototype submarine for a government prize of $50,000. A steamer carrying $300,000.00 in gold has sunk off the Uruguayan coast at a depth of nearly 3 miles, far below the current technology's ability to reach. The Advance, the Swifts' new sub can dive that deep and a new hard shell diving suit carried by the sub can work outside at the 15,000 foot depth. A rival sub building firm plans to beat Tom to the sunken gold. Tom and company overcome the rival divers to save the gold (Tom Swift Unofficial Home Page).