She hasn't been around since season 3. Now, she's the Chief of Police in Suffolk County and needs her old partner Danny's help to catch a tricky serial killer causing trouble in her town.
There's also some excitement with Erin, who might be taking a big step in her career by running for New York's District Attorney.
Even though we don't know all the details yet, it's expected to be a big, emotional ending as Blue Bloods gives a proper farewell to the characters we've loved for almost fifteen years.