'The Flash' Episode 100 Dives Into the Show's Best Use of Time Travel Yet

The Flash pulled out its most extreme and awesomely fun instance of time travel yet for its 100th episode, and it’s a whirlwind tour of the series greatest moments from (almost) every past season. There’s no better way for The Flash to celebrate 100 episodes of craziness than seeing Savitar, Zoom, and even the original Reverse-Flash from Season 1, all through the organic premise of traveling through time to fashion the craziest and most powerful weapon ever.

Episode 100, “What’s Past is Prologue,” aired Tuesday, December 4. After Team Flash discovered Cicada’s real identity last week, their grand plan is to develop a weapon that can counteract Cicada’s power dampening dagger.

Full spoilers follow for The Flash Season 5, Episode 8 — number 100 for the series — “What’s Past Is Prologue.”

To do that, they need an impossible metal and technology they don’t have, but Nora comes up with a plan so brilliant and ludicrous that it’s perfect for this, the 100th episode.

They come up with the idea get a shard from Savitar’s suit (end of Season 3) and Zoom’s Speed Force Transmitter (back-half of Season 2). Then, by infusing it with dark matter energy from the original particle accelerator explosion that kicked the entire show off, they can make a device that can negate Cicada’s power-negating mega-dagger.

When the Speedforce Transmitter gets broken in a confrontation with Zoom, Barry and Nora accidentally travel back to the events from one of Season 2’s best episodes. In “Flash Back,” Barry went back in time to learn about Tachyon’s from Eobard Thawne.

Our present Barry visits Thawne that very same day to get help repairing Zoom’s device, and it’s a total delight seeing actor Tom Cavanagh (who directed this episode) sink back into the true form of Thawne-Wells as he channels his best Hannibal Lector impersonation with just a touch of Steve Jobs.

Eventually, things work out as they always do, and it’s a total delight to see the original particle accelerator play out again with all the additional scenes. There’s professor stein with the Firestorm Matrix and DeVoe with his own device, both of them blasted by the energy on that one fateful night.

The real tragedy here is that in the final fight, Cisco, rather than breach Cicada’s dagger to an alternate Earth — like Earth-15 the “dead Earth” — sends it into outer space … and Cicada’s still able to recall it.

So there’s ultimately no real point to this otherwise totally awesome journey. At the very least, we end on a heartwarming scene between Barry and Nora as they watch Barry’s parents years into the past. Yes, The Flash went to that kind of painstaking detail to address every crucial scene in the show’s history, and it paid homage to five years of superhero TV way better than Arrow did.

“What’s Past Is Prologue” as an episode title ultimately becomes a clever and poignant way to riff on a Shakespeare quote about history, spinning it into a reference about predestination paradoxes. Essentially, what’s going on right now in The Flash influenced events all the way back at the very start of the show.

Yet at the same time, Nora’s diary entry reveals her theory that the timeline is, in fact, malleable. That contradicts everything Barry and the rest of us learned after Flashpoint, but the even more troubling thing we learned is that Nora’s been communicating with a future, imprisoned version of Eobard Thawne-Wells in 2049.

So what’s really going on between Nora and Thawne? Why is she really in the present day? It looks like “Elseworlds” is about to interrupt everything next week, and it’s bound to permanently change the universe.

The Flash airs Tuesday nights on The CW at 8 p.m. Eastern, but next up is the annual Arrowverse crossover. “Elseworlds” begins with The Flash on Sunday, December 9 at 8 p.m. Eastern, continues with Arrow on Monday at 8 p.m., and concludes with Supergirl on Tuesday at 8 p.m. Eastern.

Here’s the latest “Elseworlds” trailer that shows even more of the totally badass Batwoman.