It's been three years since Hank Voight was arrested for harassment and dishonorary behavior. Lieutenant Matthew Casey has been happy and safe ever since. He had long since ended his relationship with Haley and started dating Gabriela Dawson about a year and a half later. Casey hadn't known that Voight would get out so soon though. And when Chief Wallace Boden found out he informed Dawson's brother, Antonio that he was out. 

Antonio asked Casey if he wanted to be put in protective custody, but he declined. Said he was going to be okay. Antonio, however decided to have two undercover detectives keep an eye out for Casey and his sister. 

Casey knew Antonio placed undercover detectives to watch him and Dawson and at first he was upset but he knew Antonio only had their best interests at mind. So he let it go.  The whole firehouse knew about what was going on and they all wanted to help. Chief Boden told them all to just support Casey just like before.

But no one expected the enemy to come from within.