It’s a good thing Peter Lenkov’s muse rides shotgun. Lenkov, the executive producer of "Hawaii Five-0," was stuck in Southern California traffic last spring when he dreamed up the plot of the TV show’s 100th episode. A veteran writer, Lenkov had wracked his brain trying to find something special for a milestone that most series never get to experience.

But somewhere on Highway 101, he decided to turn his CBS drama upside-down.

When he wrote the episode, which airs at 8 p.m. Friday, Lenkov made his characters do things they had not done during the show’s five seasons and say things they would just never say.

He thought of the pilot he used to reboot the series in September 2010: What if he didn’t kill Steve McGarrett’s father as the episode opened? What if Danno, the curmudgeonly New Jersey transplant, was still married?

He calls it his "What if?" episode.

"I think fans are going to get to see the characters in situations they have never seen before," Lenkov said. "That includes their attitudes and their perspectives on other things. When we met these characters they were all broken toys. They needed each other. What if they didn’t need each other? What if their lives were complete. It’s really a very different episode."

The episode is titled "Ina Paha," which is Hawaiian for "If Perhaps." CBS offered fans only hints at what’s to come: McGarrett is kidnapped by his nemesis, the terrorist Wo Fat, and experiences an altered reality while being tortured.

But the list of recurring stars hints at possibilities: Will Yun Lee as human trafficker Sang Min, William Sadler as John McGarrett, James Marsters as Irish terrorist Victor Hesse and Larisa Oleynik as CIA analyst Jenna Kaye. (The elder McGarrett, Hesse and Kaye were all killed off early in the series.)

Also appearing in the episode is musician Gavin Rossdale of the band Bush, which played at the show’s Sunset on the Beach season premiere. He stars as "the nefarious Johnny Moreau."

"I really wanted to tell the pilot in a different way through these flashbacks," said Lenkov. "There is something in every scene."

Lenkov said the 100th episode is a reward for fans who stuck with the show from the start. It’s a bookend for the pilot, he said. From location to set dressing, some scenes are mirror images of the pilot — but with a twist.

There’s even a fun wink at the original series pilot in 1968, but only sharp-eyed fans will catch it.

"If you jump into the show, some of this story won’t make sense," he said. "I think it is still an entertaining episode but if you know the show like most of the people who have watched this from the start, this is a treat. You will get all the inside stuff."

Lenkov wanted to give his actors something, too. After nearly 100 episodes, he felt they needed a way to break out. And everyone gets a shot.

"I really wanted to do something that really allowed the actors to enjoy the episode," he said. "I really wanted them to do something fresh."

So how will Lenkov follow up an episode as big as the 100th?

For Episode 101, which airs Nov. 21, he brought back Carol Burnett as McGarrett’s Aunt Deb and gave her character a fiance — singing legend Frankie Valli.

» Find “Hawaii Five-0” episode recaps on Wendie Burbridge’s “Five-0 Redux” blog on honolulupulse.com.