Will Callen Have To Find A New Partner In The Season 9 Premiere Of NCIS: Los Angeles?

Plus, Hetty announces her retirement in "Party Crashers."

The Office of Special Projects gets a major shake-up in the Season 9 premiere of NCIS: Los Angeles, as Hetty ( Linda Hunt) files for retirement and then disappears.

WATCH: Preview Of The NCIS: Los Angeles Season 9 Premiere

In light of the Operation Manager's sudden departure—and amidst a dangerous nuclear threat—Executive Assistant Director Shay Mosley ( Nia Long) arrives on the scene to oversee the LA team and promptly begins making staffing changes.

Meanwhile, Sam ( LL COOL J) continues to mourn the loss of his wife, which we saw in the last two episodes of Season 8 (the full episodes, "Uncaged" and "Unleashed," are available to stream on CBS All Access). As a result, he tries to convince Callen ( Chris O'Donnell) to find a new partner in the new episode entitled "Party Crashers."

STREAM: Full Episodes Of NCIS: Los Angeles On CBS All Access

Will Callen heed Sam's wishes? And just where has Hetty gone off to—and why?

Watch the Season 9 premiere of NCIS: Los Angeles on Sunday, Oct. 1 at 9:30/8:30c on CBS and CBS All Access.