Two men named Johannes Niels and Merkus arrive in search of diamonds in a volcano. Tarzan guides them on condition that he can take one for Jane. Worried about Tarzan, Jane and her friends follow his trail. As Johannes and Merkus turn on Tarzan and trap him with Jane and Archimedes, they struggle out of the erupting volcano and escape the flowing lava.
- When Jane goes to the trading port, there is a sign post that has the date 1912.