Let’s start at the beginning by exploring Jesse Spencer’s exit from the show.

In the series, Casey left 51 in order to move to Oregon to take care of his

former 51 colleague and friend Andrew Darden’s sons, becoming their

legal guardian. This exit storyline left the door open for Casey to return to the show, which he’s done several times since Spencer’s departure.

Spencer’s last official episode as a series regular on the show came in Chicago Fire’s 200th episode, “Two Hundred,” in which Casey made the decision to leave Chicago in order to take care of the Darden boys upon learning that the siblings were in jeopardy of being split up via the foster care system. The episode explained Casey’s exit away in a clear manner, but the reason the storyline was introduced is because of Spencer’s desire to take a step back from the series after 10 years on the show.

As he told People following his exit, “This was a difficult decision because I have loved the show from the start but there are other things that I would like to do in the future, and there’s some family that I need to take care of. I was marking off these milestones and looking to the future … and it was time.” Fortunately, Spencer left on good terms and while “Two Hundred” might have been his final episode as a series regular, he’s returned several times since exiting the show early in season 10.