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An above-average entry in the long-running Hopalong Cassidy Western series, the enigmatically titled Mystery Man opens with Hoppy (William Boyd), California Carlson (Andy Clyde), Jimmy Rogers, and the Bar 20 cowboys driving a herd of cattle to the Circle J. Ranch, whose owner, Tom Hanlon (Bob Burns), is to pay cash on delivery. In the town of Holbrook, however, the Bar 20 crew interrupts a bank robbery committed by the notorious Trilling gang whose boss (Don Costello) remains a mystery man. Not even the shrewd Hoppy is able to detect anything wrong with the inquisitive dude, who commends him for foiling the robbery. The dude, alias Trilling, manages to spring his henchmen from jail and then concocts a plan to steal the Bar 20 cattle en route to the Circle J. Despite strong resistance from Hoppy and his friends, Trilling does manage to drive the cattle to the Circle J where he presents himself as Hoppy. The real Hopalong Cassidy, meanwhile, is imprisoned by Sheriff Newhall (Taylor Homes), who accuses him of being Trilling. But the sheriff's daughter, Diane (Eleanor Stewart), who once rescued Jimmy from a would-be killer, knows otherwise and manages to free the Bar 20 cowboys. With the sheriff's posse hot on their trail and Diane acting as a decoy, Hoppy and his men force the Trilling gang into a box canyon and a final shootout.