The plot concerns the theft of gold from a mining camp in Montana. At first the case is being worked on by Fenton Hardy but when he becomes injured he invites his sons Frank and Joe to join him. After overcoming many obstacles the Hardy boys recover the stolen gold and turn the outlaws over to the Sheriff. Below from wiki -

While their father Fenton Hardy is in Montana working on a case of missing gold the Hardy Boys go skating on Shallow Lake with their friends Chet Morton and Jerry Gilroy where they meet Jadbury Wilson, a poor old man who is also a former gold miner from Montana. Wilson tells them his tale of how Bart Dawson, a trusted member of his party, ran off during a gun fight taking four bags of their gold with him for safekeeping but failed to return once the gunfight ended. Upon returning to the Hardy home, with the injured Jadbury Wilson, the Hardy boys receive a telegram from their father requesting that they join him in Montana immediately. After a stranger, claiming to be Mr. Hardy's lawyer, ushers the boys onto the wrong train, resulting in their being kidnapped and later escaping, the Hardy boys decide to continue their journey in disguise, finally arriving at Lucky Bottom, Montana to meet with their father. He tells them that they are looking for gold which Bart Dawson claims was stolen from him by an outlaw named Black Pepper and his gang. The next day, while searching for the gold, the Hardy boys are attacked by the gang of outlaws; however they manage to escape and return to town for the night. The following day they manage to catch one member of the gang who gives them information about Black Pepper. Upon searching, they discover a cabin used by Black Pepper, and there they find a map to the hidden gold, indicating it is buried in The Lone Tree mine. While exploring the mine the Hardy boys are attacked by a pack of wolves that chase them down another shaft which happens to be the main mine entrance and the first landmark on the map they found in Black Pepper's cabin. Following the map, the Hardy boys recover the gold which leads to them being attacked by Black Pepper himself. They manage to get the upper hand and march back to town with both the outlaw and the gold, to later learn that Bart Dawson had believed Jadbury Wilson was dead and now that he knows differently he is willing to share the recovered gold. Bart Dawson explains that he had not returned with the gold immediately following the first gun fight because he suffered from amnesia caused by a gunshot to his head.