The first three-hour crossover in more than five years unites the crews from 'Chicago Fire,' 'Chicago P.D.' and 'Chicago Med.

find themselves on death row together and fight for the fame that will keep them from the gallows in 1920s Chicago.

But here’s what really makes it special: Chicago Fire doesn’t rely on the same cast forever to stay good. The lineup has changed as often as the coffee supply, but the show keeps thriving.