Hoppy's ranch is threatened by rustlers. Hoppy and the gang oblige as usual.

After Hoppy helps round up the Trilling gang, Trilling breaks his men out of jail and they rustle the Bar 20 herd for revenge. When Hoppy gets the herd back, Trilling posing as a Marshal not only gets the herd again, he gets the Sheriff to jail the Bar 20 gang. Getting help from the Sheriff's daughter, Hoppy and pals start out once more after Trilling.—Maurice VanAuken <[email protected]>

The mystery man of the title is Bud Trilling (Don Costello), notorious leader of a gang of bank thieves and cattle rustlers, whose fame is that he has never been seen by his victims. Hoapalong Cassidy (William Boyd) and his Bar 20 pals, California Carlson (Andy Clyde) and Jummy Rogers (Jimmy Rogers), are in Holbrook when Trilling's gang hold up the bank, but so clearly does Trilling disguise his connections with the gang, Hoppy does not suspect him. Overconfident, Trilling decides to rustle the Bar 20 cattle that Cassidy, for the Bar 20 owner, is planning to sell in Texas, and this decision finally leads to Trilling's arrest and the end of his activity.—Les Adams <[email protected]>