Fool's Gold (1946)

In the 56th film of the 66 in the series (and the first one produced by William Boyd Productions instead of Harry Sherman), Hopalong Cassidy undertakes to help out his old friend, Army Colonel Jed Landry. The Colonel's hot-tempered son, Lieutenant Bruce Landry, has struck his commanding officer and is facing a court martial. Bruce deserts and is believed hiding at a robber's hideout-town known as Twin Buttes. Cassidy departs without telling his two pals, Lucky Jenkins and California Carlson, but they learn his destination and follow him. Hoppy arrives at Twin Buttes and attempts to register at the inn ran by Jessie Dixon but she is suspicious and refuses him a room. Cassidy makes friends with "Professor" Dixon, Jessie's father, and is given a room. Bruce is being kept in a hideout by a group of holdup men headed by Dixon, and has fallen in love with Jessie. She advises him to give himself up and face the Army court-martial, rather than participate in a series of shady deals engineered by her father. Cassidy escapes a trap set for him by Dixon and meets Lucky and California and they decide to look over Dixon's "laboratory" in the mountains. They find the place is a front for a plant where the gang makes copper bricks covered with a thin coating of gold. They ride fast to catch up with the gang when they learn a gold shipment is to be held up and the fake bricks substituted.