Hawaii Five-0 will spend its 150th episode digging into the show's mythology, and that means taking on a mystery that didn't quite die in the series 100th hour.

Steve McGarrett (Alex O'Loughlin) killed Wo Fat in "Ina Paha (If Perhaps)," finally putting an end to his pseudo- brother's reign of terror. It's hard to believe that the milestone episode aired nearly two years ago, but it's certainly not difficult to believe that fans still have questions regarding Wo Fat's past. Thankfully the criminal's parents, adoptive or otherwise, will be on hand to answer a few questions this fall.

Dae Won (Sung Kang) and Doris McGarrett (Christine Lahti) will both feature in episode 7, the latter making her first appearance since season 3.

"It's really a mythology episode," showrunner Peter Lenkov told TV Line. "It will tie up some loose ends and spark a new storyline in the episode that follows."

Catherine Rollins (Michelle Borth) will also appear, but she won't be in town to rekindle her romance with Five-0's leader.

"People are interpreting it like her coming back into the relationship and starting all over again, but I just don't think that can happen after all that these two have gone through," Lenkov told Entertainment Weekly. "Her relation to the story is more plot driven than reigniting that relationship."

Falling Skies Sarah Carter might also drop by Oahu, which should be interesting considering she plays McGarrett's current love interest Lynn.

"We're trying to get Lynn get in the episode also," Lenkov explained. "We'll hopefully get her back."

Are you looking forward to the 150th episode, or do you think the Wo Fat storyline should be left in the past? Sound off in the poll below!

Hawaii Five-0 will return to CBS on Sept. 23 at 9 p.m. ET.

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