Spoilers for the first three seasons of Sons of Anarchy coming.
After the abduction of Jax' son Abel, the club needs to get him back. Unfortunately, it soon turns out that the IRA brought him to Belfast, where the club can't go because of the bail they posted after their attack on Zobel's church back in season 2 - before the trial is over, no one is allowed to leave the state. The trial will in all likelihood lead to a long sentence for the members involved, including Jax, which means no chance at finding Abel. Jax takes desperate measures, striking a deal with Agent Stahl, who's still prosecuting them but would prefer the career-making case of taking on the IRA. Of course, under the rules of the club, working with the police carries a death sentence under all circumstances.