Huns Within Our Gates (1918)

During World War I, Arthur Morgan, an inventor from the South who has perfected a revolutionary airplane engine, needs money to complete his work because his family fortune was lost during the Civil War. A German spy offers to buy his invention, but Arthur refuses, after which the spy tries to capture Arthur's sweetheart and then seduce him through the wiles of the Countess Von Shoenburg. When the spy finally resolves to destroy Arthur's workshop, Rover, the family dog, carries the bomb away and drops it by the tree behind which the agent is hiding. The spy is killed and the ground opens, revealing Arthur's long-lost family fortune.