A prosecutor in the historical city of Sandomierz investigates two gruesome murders that appear to be based on the medieval legend of the Blood Libel (that the sacrifice of a Gentile baby is needed to make matzo for Passover). To understand this film, one needs to know that there are not one but two churches in Sandomierz that have wooden panels that depict the Blood Libel. The painting in the Cathedral hidden behind a curtain forms part of the plot. The murderer uses the legend and people's need to believe in fantastic conspiracies to deflect suspicion. Before condemning Poles for belief in the unbelievable, consider the persistence of Birthers in North America.

The second missing bit of background is that the Polish system of law resembles the French system where an investigating prosecutor leads the investigation rather than a police officer, as in common law jurisdictions such as the United States and the British Commonwealth.

Ziarno Prawdy is well written and acted and provides some insight into latent anti-Semitism and the need for a bogeyman to blame for evil. I would give the movie an extra star if it didn't exhibit the Polish need to have extra-gruesome murders for its mysteries.