Danny never hung out with the Tanglewood boys, though Louie did, instead he followed in his grandfather's military footsteps and joined the Marines BEFORE 9/11. After the attacks on the Towers, he was sent overseas where he's injured. This results in him being given a medical discharge. Still wanting to do good, to protect and serve, he joins the NYPD and becomes a CSI. How does this change effect his career and what role does it play in his relationship with Mac?