Booth (David Boreanaz) is out to kill Zack (Eric Millegan) for daring to hurt his wife in the upcoming final season of "Bones."

The final installment of the long-running FOX series is expected to feature the return of Brennan's (Emily Deschanel) former intern. Zack was a brilliant forensic anthropologist who was chosen to become Brennan's assistant in the Jeffersonian. A new extended trailer for the series' final season shows Zack's past before he became embroiled in the messy case involving the serial killer Gormogon (Laurence Todd Rosenthal).

In the clip, the people at the Jeffersonian are shown remembering what Zack was like when he was working with them.

Hodgins (T.J. Thyne) says the young doctor fondly called himself the "King of the Lab." They were best friends in the past and Hodgins was one of those people who still could not believe that Zack was working with the cannibalistic criminal. Zack was advised to plead guilty so that he would not go to prison. Instead, his lawyers made a case of him being psychologically unsound and the court decided he would be sent to serve time at the McKinley Psychiatric Hospital. Zack loved working for Brennan and once thought he was in love with her. She was heartbroken when it was revealed he was Gormogon's inside man.

During the last finale, Brennan was at her office when Zack came to abduct her. According to the promo photos released, Booth is extremely worried that he may not see her alive again. The FBI agent is convinced that Zack means her harm and is prepared to kill the other man once he tracks him down. The snaps, however, reveal that Zack may not be the serial killer, the Puppeteer. Brennan appears to be stopping her husband from killing her former apprentice. If it is not Zack, then who is the monster they are looking for?

The last season of "Bones" will premiere on Jan. 3, 2017, 9 p.m. EST on Fox.