The top scripted series on each of the Big 4 broadcast networks—Grey’s Anatomy (ABC), FBI (CBS), Chicago Fire (NBC) and 9-1-1 (Fox)—all have something in common: they are the motherships of drama franchises that stage frequent crossovers. Building integrated universes consisting of multiple series, whose characters move seamlessly from one show to another, has become a broadcast staple and arguably the most successful storytelling formula on network television over the past decade that continues to draw viewers amid dwindling linear ratings.