While young Tom Swift is in a jewelry store shopping for a ring for Mary, he meets a man who claims to be willing to teach Tom how to make diamonds. Later, the man (who happens to be one of the party that Tom rescued in the book Tom Swift and his Wireless Message) tells Tom his story — how he was approached by a group of men who knew how to make diamonds, how he actually watched them make diamonds, and how he gave them some money, and how they dumped him (but not before giving him a fortune in diamonds). The man urges Tom to go with him on a hunt for these diamond makers.

After some thought on the matter, Tom agrees to go after the diamond makers, taking with him Ned, Mr. Damon, and Mr. Parker (the doomsday-predicting scientist in Tom Swift and his Wireless Message who predicted the destruction of the island). After a long search and plenty of adventures along the way, the small band locates the diamond makers — but is then captured.

Can they escape from Phantom Mountain? Can they learn the secret of the diamond makers? Will the mountain really be destroyed, as Mr. Parker predicted? It's all there, in Tom Swift Among the Diamond Makers.