Season two of Hawaii Five-O will be chock full of guest stars, from Tom Sizemore to William Baldwin to Greg Grunberg.

But the CBS is making no secret of the exciting casting scoop of them all: the return of Terry O'Quinn to the small screen. This beloved Lost star will play a key role from the September 19 premiere, coming on board as the Navy Seal Lt. Commander who trained Steve McGarrett and now must save Steve McGarrett from prison.

Check out the first official trailer for the second season and check out our Hawaii Five-O spoilers section for more intel.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.