In the trailer for upcoming documentary “The Sentence,” a young girl receives a phone call from her mother. Before they are connected, an automated recording plays: “This call is from a federal prison.”

“The Sentence” sees director Rudy Valdez telling the story of his sister, Cindy Shank, who received a 15-year mandatory minimum prison sentence for “conspiracy charges related to her deceased ex-boyfriend’s crimes.” When asked if she knows why her mother is incarcerated, one of Cindy’s daughters replies, “The kids that lived with her were doing bad things and that’s why she’s [in prison,] because she was actually with those kids.”

The situation isn’t quite that simple, but the young girl isn’t that far off the mark. Cindy was basically convicted of living with a drug dealer. “I didn’t sell these drugs. I didn’t buy these drugs,” she explains over the phone. “I’m the girlfriend. Any crimes that he committed while we lived together, I was charged with.”

It’s an outrageous miscarriage of justice made even more horrifying by the knowledge that Cindy’s isn’t a unique case. “There are thousands and thousands of people just like her, serving the same ridiculously insane sentences,” a commentator reveals in the spot.

For Cindy, the real tragedy of it all isn’t even prison. “Missing my daughters grow up: that’s what I was sentenced to,” she declares.

“The Sentence” debuted at Sundance this year. The doc opens in select theaters October 12, and will air on HBO October 15.