The Boondock Saints Film Series is a vigilante-justice film series based on the lives of the brothers MacManus--Connor and Murphy. The two currently released films rely heavily on the plotline revolving around the planned killing of various mafia soldiers, bosses, and underbosses, as initiated by the self-defense killing of Ivan Checkov, a Russian Mafia soldier, and the setup by the Italian Mafia of their best friend, Rocco, to be killed by the Russian Mafia.
After these events, the two brothers and Rocco decided to begin vigilante efforts to take crime levels down with their own hands; eventually, this leads them to execute Mafia Don Papa Joe. Afterwards, they flee to Ireland, and do not return for several years. After several years, the boys return due to the gruesome killing of a beloved priest in Boston.
This initiates another serial killing by the brothers: however, as it ends with the killing of The Roman, the boys almost willingly go to prison after their last splurge of vigilante efforts. However, it's strongly implied that the third film will release them from prison and set them back to work, again hunting the world's finest criminals.