A true-crime podcaster receives an unmarked package on his doorstep. Inside: five cassette tapes labeled like episodes—and a note that reads simply, "You're ready."

Marcus Cole has spent six years hosting Cold Trail, a podcast dedicated to unsolved cases. What his audience doesn't know is why he's really obsessed with cold cases: his mother was murdered when he was six years old. Stabbed seventeen times in their home. No witnesses. No forensic evidence. The killer was never found.

The first tape contains the audio recording of that night.

His mother's voice. His six-year-old self being tucked into bed. And then the sounds of someone entering the house—someone she knew, someone she trusted—followed by four minutes he can never unhear.

But this isn't just evidence of an old crime. As Marcus listens to the remaining tapes, he ...