Sean Reagan will be accompanying his father, Danny, to Boston in "Boston Blue," but the series has recast the "Blue Bloods" spin-off's role - here's why.

Andrew Terraciano will not return as Sean Reagan in the spinoff, and Mika Amonsen will now take his place in 'Boston Blue'

Blue Bloods got viewers invested in the Reagan family — but how are the characters related to each other? The CBS series, which premiered in 2010, followed the lives of the Reagan family, many of whom worked for the NYPD under the leadership of NYC police commissioner Frank Reagan (Tom Selleck).