The Flash's 100th episode took fans on a trip down memory lane -- and quite a lot of familiar faces returned in the process.
Spoilers for tonight's episode of The Flash, "What's Past is Prologue", below!
The episode saw Team Flash coming up with a weapon that could potentially defeat Orlin Dwyer/Cicada (Chris Klein), but with one major catch. Barry Allen/The Flash (Grant Gustin) and Nora Allen/XS (Jessica Parker Kennedy) realized that in order to get the supplies that the weapon needed, they would need to gather materials from different points in the past, all without messing up those events.
Much like Arrow's 100th episode, this allowed the show to revisit some of its past allies and foes, all in a pretty unique way. So, who made a return appearance in the episode? Read on to find out.
The first event that Barry and Nora traveled to was the Season 3 finale, in which Team Flash defeated the evil Barry Allen, also known as Savitar.
Those original events - particularly Barry phasing to free Savitar from his suit, and Iris ultimately shooting and killing him - played out as they did the first time, allowing Nora to successfully get away with a small fragment from Savitar's suit.
The episode then traveled to the events of Season 2's "Versus Zoom", where Hunter Zolomon/Zoom (Teddy Sears) tried to kidnap Wally West (Keiynan Lonsdale), as a way to force Barry to give up his speed. In that whole ordeal, Zoom injected himself with a vial of Velocity 9, which present-day Barry and Nora wanted to pick up once he'd discarded it.
Again, the scene played out similarly to how it did way back in Season 2 -- until Zoom detected Barry and Nora's presence, chasing them through the Speed Force.
Oh, and in the meantime, present-day Barry had an awkward run-in with Harry Wells (Tom Cavanagh), the Earth-2 counterpart who helped Team Flash both in Season 2 and Season 4.
So as to not mess up the timeline, Barry pretended to be his former self to Harry, and it generally seemed to have worked.
Thanks to Zoom yanking Barry out of the Speed Force, he and Nora ended up in the events of Season 2's "Flash Back", in which that Barry revisited the events of Season 1.
(I know, it's confusing.)
With no other way out, Barry and Nora were forced to approach Eobard Thawne, from back when he was pretending to be Harrison Wells. Wellsbard agreed to help, but the whole exchange seemed pretty darn shady. (Thankfully, the final season essentially explained why.)
We also saw Wellsbard in the next point in time that Barry and Nora traveled to -- the night of the particle accelerator explosion, all the way back in the show's pilot.
Of course, as fans learned in Season 4's "Therefore I Am", Clifford DeVoe/The Thinker (Neil Sandilands) and Marlize DeVoe/The Mechanic (Kim Engelbrecht) were present on the night of the Star Labs explosion, hoping to harness some of the particle accelerator's energy to increase Clifford's cognitive abilities.
While we don't see The Thinker's origins played out again, the fact that they can be seen in the crowd during the 100th episode's replay of it proves that #ItsAllConnected.
Sadly, Robbie Amell doesn't really get a chance to do anything new as Ronnie Raymond/Firestorm, as the 100th episode largely uses archival footage of him from Season 1.
Still, that doesn't make his presumed death in the particle accelerator explosion -- as well as his return and sacrifice as Firestorm -- any less heartbreaking.
In one small moment, the 100th episode answered a question that some fans have had since the pilot episode -- what did Gorilla Grodd look like pre-transformation?
The pilot briefly showed Grodd's mangled cage, hinting that the villain would eventually appear in Season 1 and beyond. And in this revisit of the particle accelerator explosion, we briefly saw a regular-sized Grodd in his cage.
And of course, Martin Stein (Victor Garber) was also present at the night of the particle accelerator explosion, something that would ultimately bond him and Ronnie into Firestorm for the first time.
Because of that, we got a brief appearance from Stein, who has held a special place in the Arrowverse ever since, thanks to his (albeit a little tragic) role on Legends of Tomorrow. Considering the fact that this week's Legends had a homicidal spirit possess his puppet counterpart, it's a pretty big week for Martin Stein fans.
That montage of people getting affected by the particle accelerator also briefly showed Clyde (Chad Rook) and Mark Mardon (Liam McIntyre), two brothers who served as bank robbers in Central City before the accident happened. Both brothers were given weather manipulation powers, and became the first ever villains of the week that Barry fought.
This marks the second week in a row that Mark Mardon has appeared on The Flash, after his relationship with his estranged daughter served as a subplot for last week's "O Come, All Ye Faithful".
And for one more emotional punch in the gut, Nora traveled back even further in time, to right before her namesake, Nora Allen (Michelle Harrison), was killed by Eobard Thawne.
Nora, Barry, and the audience got to see a sweet moment between Nora and Henry (John Wesley Shipp), as they stood on their front porch and talked. But given what we know happens next (and what we know Nora has gone to the past to discover), it only makes the moment a bit more heartbreaking.
And of course, the episode's "post-credits scene" of sorts gave fans a completely different kind of plot twist, one that Supergirl and Arrow viewers have already seen.
The scene shows The Monitor (LaMonica Garrett) destroying what is referred to as "Earth-90", and is inhabited by John Wesley Shipp's version of Barry Allen.
There are a slew of characters teased in that brief sequence, which you can read all about here. And as for what comes next...you'll have to watch next week's "Elseworlds" crossover to find out.