On a cattle drive Hoppy, camp cook Windy, companion Lucky, and young Artie Peters encounter an eccentric professor. The professor professes to be searching for the evolutionary missing link, but in reality he is a cattle rustler who uses his dynamite to scatter the cattle in order capture some of them. Hoppy and Bar 20 guys ultimately capture the professor.
William Boyd
as Hopalong Cassidy
George 'Gabby' Hayes
as Windy Halliday
Russell Hayden
as Lucky Jenkins
Nora Lane
as Nora Blake
Harry Worth
as Horace Hepburn
Lois Wilde
as Laura Peters
Billy King
as Artie Peters
William Duncan
as Buck Peters
Jack Rutherford
as Blackie
Ernie Adams
as Keno
John Beach
as Pete
Ben Corbett
as Henchman
