The Painted Hills (1951)

After years of prospecting, Jonathan finally strikes gold. He returns to town only to discover that his partner has since died and left Tommy fatherless. He decides to leave Shep (played by Lassie) with Tommy to cheer him up. Meanwhile, Jonathan's new partner, Lin, isn't interested in sharing the gold, and lures Jonathan to his death. Lassie immediately deduces what's happened, so Lin poisons Lassie. Lassie barely pulls through and pursues Lin to a climactic confrontation where, due to an off-screen accident with some liquid nitrogen, Lin's gun jams.