Someone wants to kill Magpie Harper. Crash and Dusty arrive too late, Magpie Harper is allready dead.
When Moreland's henchman attack Bob Horner the Range Busters break it up. Horner dies but they get the baby Moreland was after. He must have it to become it's guardian and take control of the Horner ranch. When the henchmen catch up with Alibi, they take the baby cradle not realizing Alibi has substituted his dummy Elmer. Crash having evidence Moreland is the one they want, now has a plan to expose him.
The Canadians have discovered a valuable substance called Compound X, which can cure infantile paralysis. When a country at war with Canada learns that Compound X also contains magnetic properties that could aid them in their warfare against the British, they send agents to infiltrate Canada and steal a large quantity of the substance. It's up to Sgt. King (Allan Lane) and his Mounties to track down the agents and put an end to their scheme.
