The Explorers look forward to finally going on the canoe trip that they had to postpone when they went to Peru, but Hap Livingston is called to Columbia to investigate an emerald mine. The boys are disappointed, especially as Hap would take them along if he could afford it. By a stroke of good luck, Jack and Ken notice a strange man digging in the cemetery one night. They investigate and discover an explosive substance. The boys receive a reward from the community which they decide to use so that they can go to Columbia with Hap.

When the Explorers arrive in Columbia, Mr. Corning fails to meet them. The group travels to the mine hoping to find him there. During their trip, several people try to discourage the group from traveling to the mine. Upon their arrival at the mine, they are told that Mr. Corning is missing and is presumed dead!

Mr. Rhodes has taken charge of the mine and asks the group to leave. He reveals that Mr. Corning is possibly a prisoner of a notorious bandit and that there is little hope of finding him alive. The Explorers suspect that Rhodes is keeping the mined emeralds for himself and is responsible for whatever has happened to Mr. Corning. In spite of the danger, the group stays at the mine, facing many dangers before they finally solve the mystery.