Twelve will face on his toughest episodes to date in Doctor Who's "Heaven Sent."

'Doctor Who' Season 9: Peter Capaldi & Steve Moffat Tease New Companion, Who Will It Be?

Season 9's penultimate episode will be unlike anything the Doctor or his fans have ever experienced. As the Who universe gears up for a return to Gallifrey and Clara's departure, the BBC will send the Doc on a solo mission.

"Trapped in a world unlike any other he has seen, the Doctor faces the greatest challenge of his many lives. One final test. And he must face it alone," the BBC teased. "Pursued by the fearsome creature known only as the Veil, he must attempt the impossible. If he makes it through, Gallifrey is waiting..."

'Doctor Who' Season 9: Gallifrey Stands! Twelve To Head Home In The Finale

Showrunner Steven Moffat weighed in on the upcoming ep, which will air just weeks after Doctor Who made their first foray into the world of found footage storytelling.

"It's Capaldi on his own," Moffat told Entertainment Weekly. "It's one of the strangest, and most atmospheric, and creepiest episodes that we've done, with the Doctor monologuing to stay alive."

Clara's death will weigh heavily on Twelve as "Heaven Sent" kicks off. Will the Time Lord find someway to see his friend once more? Find out when Doctor Who airs Saturdays on BBC and BBC America.