LOS ANGELES — Here on the set of the new prequel “Fear the Walking Dead,” a mother, her son and her boyfriend are crammed together in the cab of a beat-up Ford truck, terrified that something’s not quite right in their city.

After the pickup careens out of a tunnel to the Los Angeles River, the mother frantically tries to call her other child, while the sullen teenager twirls the radio dial, fruitlessly seeking information about the disturbing sights they’ve witnessed.

“Nobody’s talking about this,” he says with disbelief. “Nobody’s saying anything.”

Come Aug. 23, when “Fear the Walking Dead” has its premiere on AMC, they will.

“The Walking Dead” has more than 20 million weekly viewers, and this companion series enables the network to dip back into its enormous, devoted fan base, this time with a dark family drama. It will explore some of the same ideas about survival and human nature but will also stake out its own thematic terrain.