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In a sleepy northern Italian town on November 26th, 2010, Yara Gambirasio disappears without a trace. In February, after a terrible winter of fruitless searching, her body is found, and the facts of her brutal killing are established. While all of Italy mourns her death, local D.A. Letizia Ruggeri begins searching for Yara's killer, known only as Unknown Male Number 1.

Letizia Ruggeri recommits to the long-shot DNA-driven investigation into the headline-grabbing case of Yara's murder. Investigators descend on nearby mountain villages, questioning locals and collecting DNA samples. When geneticists examine the of DNA of deceased bus driver Giuseppe Guerinoni, they uncover old family secrets that may hold the key to bringing the killer to justice.

DNA samples lead police to a 45-year old man named Massimo Bossetti. His DNA is a perfect match for Unknown Male 1, and he is arrested. Italy is transfixed; the killer appears to have been found. But inside the prison cell, a different story unfolds. Bossetti is an innocuous father of three with no history of violence. And as the defence investigates the details of the case, holes begin to emerge.

A year long court-case ensues, and the fraught proceedings fuel national debate about Bossetti's culpability. New evidence and suspicions emerge, and what started out as an ironclad case for the prosecution begins to bend towards the defence. The case reaches new levels of speculation and scrutiny, as those involved prepare to learn who, if anyone, will be held responsible.