Michael Portillo's rail voyage continues through northern California, moving east in search of the state's greatest national treasures. To unearth the region's dangers deep underground, Michael visits the seismology department at UC Berkeley and hears the story of the catastrophic 1906 San Francisco earthquake. Experiencing the country's largest multidirectional shaking table in action, he learns how engineers today strive to prepare for the high risk of an earthquake.

Alighting at Stockton, Michael delves into California's pioneering agricultural technology and buckles up on a caterpillar tractor the likes of which revolutionised farming and construction equipment around the world. And following the advice of his Appleton's, Michael immerses himself in the sublime beauty of Yosemite National Park, learning about the committed 19th-century conservationists who campaigned for federal protection. He gets up close to the magnificent fauna and flora, attempts a rock climb and comes across a 'Buffalo Soldier' patrolling the wilderness.