Criminal Minds returns for its 10th season today, 1 October at 9pm on CBS.

Previously, the action-packed Season 9 second part finale ended with the departure of Alex Blake (Jeanne Tripplehorn) character from the show.

The premiere episode is titled X, and the synopsis reads:

The BAU team investigates a series of murders in Bakersfield, Calif., which have left the victims unidentifiable. Also, the team welcomes new agent Kate Callahan into the BAU fold, who assists them in their efforts in solving the cases, on the 10th season premiere of criminal Minds.

Jennifer Love Hewitt joins the cast as FBI undercover agent Kate Callahan and Kerr Smith guest stars as Frank Cowles.

Showrunner Erica Messer told TVGuide, "Episode 10.1 airs on 10/1. How cool is that?

"It wasn't planned. We didn't know that until we started back up and we were well in the swing of things, and [CBS] mentioned that the premiere would be a little later than usual because of Extant and Survivor. So it's great timing."

Agent Kate Callahan's line of specialisation will be undercover operations.

Messer said: "You see [Kate] doing something you don't see our team ever doing, which is schmoozing with a bad guy, working undercover to take a pedophile down."

You can log on to the official website for broadcast details and to watch episodes online.

You can also click here to watch the premiere episode online via live stream.

Watch the promo here.

Here are a few tweets on the upcoming season premiere: