Over a decade ago, a gruesome killer was convicted of murdering four women. Though police could never prove it, investigators always suspected there were many more victims. But the killer took his secrets with him to San Quentin. When he arrived on death row, a fellow inmate offered him protection on the yard. Over the course of a decade, the killer confided in this inmate, revealing details about 20+ unsolved murders for the first time. This inmate-turned-informant has now handed over 300 pages of detailed notes about the killer's unsolved murders to Ken Mains, an ex-FBI investigator and cold case detective. Mains will work together with the informant to uncover new victims, bring closure to families, and expose one of America's most prolific serial killers in real-time.