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