WARNING: The following feature contains SPOILERS for the 100th episode of The Flash.
Despite a great deal of excitement, The Flash's 100th episode was little more than a glorified clip-show. While the promo materials for The Flash 100th episode promised several returning villains from throughout the show's run, most of these scenes involved reusing footage from earlier episodes and cleverly editing Grant Gustin and Jessica Parker Kennedy into them. While the craft employed by the special-effects team to do this was skillfully handled and well-directed by Tom Cavanagh, the whole episode still feels like something of a missed opportunity.
The Flash 100th episode, titled "What's Past Is Prologue", centered upon Team Flash pondering the question of how to confront the serial-killer Cicada, now that they've deduced Cicada's real identity. Given that Cicada's meta-tech dagger gives him the ability to negate the superpowers of metahumans, it's all but impossible for Team Flash to approach the supervillain directly. This leads to Nora West-Allen, the Flash's daughter from the future, suggesting that they create a piece of meta-tech of their own in order to negate the powers of Cicada's dagger.
Related: Theory: Flash Season 5's Villain Isn't Cicada, It's [SPOILER]
The team quickly figures out a way to build a super-charged magnet that can hold the meta-tech knife in place and negate its powers, though gathering the materials to build it requires a scavenger hunt across history. What follows is a series of sequences in which Nora and Barry run back in time to the scenes of several of Barry Allen's greatest battles, so they can retrieve items such as a piece of Savitar's armor and the device Zoom used to drain Barry Allen's speed, before forging the meta-tech by traveling back to STAR Labs on the night of the particle accelerator explosion that created the first wave of metahumans.
We do get to see Savitar, but only just as his armor is being destroyed by the Flash. We do get to see Zoom for slightly longer, but he only manages to chase after Barry and Nora for a few moments; Zoom is then destroyed by a Time Wraith. Eobard Thawne puts in a brief cameo, but he's still in Harrison Wells body and does little more than give Cavanaugh a chance to dust off his first character in The Flash to heckle Barry Allen for a bit, all while he repairs a piece of technology that was broken during the struggle with Zoom. Amusing as the scene is, it's a far cry from the high-action anniversary special of The Flash that was advertised.
All of the episode's new content and action is largely reserved for the final 15 minutes, where Team Flash gets a chance to test their new tech against Cicada. It spoils little to reveal that Cicada escapes to menace Team Flash again in the second half of season 5, though they do discover an important limit to Cicada's powers - metahumans like Killer Frost who gained their powers through medical experiments are not affected by Cicada's powers. The only other major revelation to come of the episode regards Nora having an ulterior motive for coming to the past beyond meeting her father - gathering information for an enemy of The Flash from the future of 2049.
Ultimately, "What's Past Is Prologue" is not a bad episode of The Flash. It is, however, something wholly different than what was advertised, and it's a bit of a disappointment given what Arrow accomplished with its 100th episode. It seems that the creative team behind The Flash's 100th episode decided to bring in familiar names to provide audiences with some nostalgia while spending the rest of the episode setting up the second half of season 5, without providing audiences with anything of real substance to hold onto. Hopefully fans of The Flash can count on action aplenty next week, when the Elseworlds event begins.