Uptight and straight-laced, FBI Special Agent Sarah Ashburn is a methodical investigator with a reputation for excellence--and hyper-arrogance. Shannon Mullins, one of Boston P.D.'s "finest," is foul-mouthed and has a very short fuse, and uses her gut instinct and street smarts to catch the most elusive criminals. Neither has ever had a partner, or a friend for that matter. When these two wildly incompatible law officers join forces to bring down a ruthless drug lord, they become the last thing anyone expected: buddies.