Booth and Brennan's relationship will be challenged in the final season of "Bones."

In "Bones" season 12, Brennan and Booth's marriage will have to deal with a huge hurdle, and rumors have it that this hurdle has something to do with the return of Zack.

The season 11 finale saw a major cliffhanger when Zack, the former intern of Brennan, reemerged to kidnap her. At the time of the incident, the Jeffersonian was investigating a case involving the Puppeteer and had been having bad dreams about Zack. Through his wife's psychiatrist, Booth learned about the trouble of his wife. However, he never had the chance to talk to Brennan about it since Zack kidnapped her first. In the season 11 finale, Brennan was completely frightened upon recognizing who her captor was.

Fans expect that the upcoming season will shed light on how Zack was able to get out of the psychiatric facility he's supposed to be locked up in. But while it looks like Zack is highly dangerous, hints suggest that he might not be the culprit.

In the final season of "Bones," Brennan will be freed and she and her husband will fight over the fate of Zack.

"They're going to have very different views--and it's not necessarily what you are expecting--of Zack's culpability and how much you can blame him, how one would judge him, and how one would forgive him," said executive producer Jonathan Collier.

"Bones" season 12 is expected to premiere sometime in 2017 on FOX.