The war with the Zygons is shaping up to be one of the Doctor's most difficult challenges that he's ever faced.

In last Saturday's "The Doctor Who" episode titled "The Zygon Invasion," viewers saw the Zygons return and create havoc. The Zygons started a revolution so that they could conquer the Earth and live in it in their true forms.

The Zygons also revealed that not only could they shape shift but they could also transform themselves into the appearance of loved ones. Viewers were shocked to learn that even Clara had been cloned.

As they take on human form, the Zygons can also read the minds of potential victims so they are much more difficult to kill.

The story continues next Saturday with the episode "The Zygon Inversion."

According to the official episode synopsis, the shape shifting Zygons are everywhere in the U.K. and there is no way of knowing who can be trusted. With the UNIT neutralized and under the Zygon's control, it is up to the Doctor to save the day. Will he be able to stop the war and save his friends?

It seems that the future of the planed is sealed in a box in UNIT's back archive. The Doctor works with Osgood to retrieve the box. He discovers that the box isn't simply a box but is actually an Osgood box. According to Osgood, the box can possible end the ceasefire but it can also start a war.

Season 9 News

Steven Moffat, executive producer for "Doctor Who" also spoke about the future involvement of Maisie Williams. Moffat confirmed that the "Game of Thrones" actress would not be replacing Jenna Coleman as the next companion. However, he did reveal that Williams would be returning to the show this season.

Williams will once again appear on the tenth episode of the season titled "Face the Raven." According to writer Sarah Dollard, Ashildr will once again team up with the TARDIS team.

"Doctor Who" airs every Saturday at 9 p.m. ET on BBC America.