The worst type, Australian convicts from San Francisco called Sydney Ducks, arrived in Mariposa, a new town in the foothills of the Sierra Nevada Mountains, bringing their particular kind of brutality and villainy. To add to the gold miners’ distress, the California bandit, Juaquin Murrieta had begun attacking anyone with a white skin.

From Monterey, U.S. Marshal Matt Janzene rode to stop the trouble. Once in Mariposa, he discovered that his sister Susan was in the middle of the violence with vital information he needed. Before they can meet, her cabin explodes with her inside. Soon afterwards, Janzene receives a mysterious note indicating that she is still alive, but in bad trouble. How can she have survived the explosion? Can she still help him? Do the Sydney Ducks want her dead?

Janzene goes to find Susan, but is ambushed by the Sydney Ducks and his body is swept away down Mariposa Creek by its swift springtime currents.

But the Sydney Ducks underestimate Matt Janzene. He is too tough to kill and too dangerous to mess with. They soon find out why.