Theo's past will be revisited in the upcoming midseason premiere of "Teen Wolf" season 5 when it returns on MTV in January.

An earlier interview with series showrunner Jeff Davis revealed that the very first scene in the midseason premiere of "Teen Wolf" season 5 will provide a glimpse of Theo's (Cody Christian) past crime.

"You see the very first image [of the premiere in the trailer]," Davis explained during the 2016 New York Comic Con. "It's Lydia in the water approaching that bridge where Theo killed his sister. That's one of the mysteries, why did Theo kill his sister, that we deal with right at the beginning of 5B. It's a fun story."

Still, there is no word yet regarding the reason why Lydia (Holland Roden) was in the water.

During the first part of "Teen Wolf" season 5, Lydia was staying in a catatonic state after Theo strangled her with his claws.

This could reveal why Theo decided to cause a chaos when he returned to Beacon Hills at the start of the season.

Meanwhile, the episode synopsis for "Teen Wolf" season 5 revealed that the pack will be investigating the mysterious infection that is hurting Deputy Jordan Parrish.

According to the official synopsis for the episode titled "The Last Chimera" that will be aired next month, "Scott and Stiles try to learn what is behind the mysterious infection that is killing the town sheriff."

On the other hand, Davis revealed earlier that there will be two new enemies who will be introduced in the second half of the season. These are the Beast of Gevaudan and the Desert Wolf.

Davis explained that they normally begin a "Teen Wolf" season by dealing with different mythologies. "This was one of those more fascinating ones, the story of the ghost riders and the idea that the Hellhound, or the black dog, is part of it all the way back to Woden's Hunt," he mentioned in a previous interview with the Entertainment Weekly.

"Teen Wolf" season 5B is slated to begin on Jan. 5 on MTV.