There are over 300 million guns in the United States. Imagine having to find just one?

This is the problem rookie ATF Agent Miranda Lopez faces.

There has been another mass shooting in America, and thirteen people are dead. But this time, it’s different because one of the victims is the daughter of Marco Barros, a powerful pro-gun rights senator.

The killer is still at large. Their identity and motive remain a mystery. Miranda’s only lead is the murder weapon: an AR-15, a.k.a. the “black rifle.”