Spoilers are coming at a blur, so don't say that you haven't been warned!
Regular Cast (Plus Earth-2 Counterparts): Grant Gustin (Barry Allen/The Flash/Bartholomew Allen); Candice Patton (Iris West/Iris West-Allen), Danielle Panabaker (Caitlin Snow/Killer Frost), Carlos Valdes (Cisco Ramon/Reverb), Tom Cavanagh (Dr. Harrison Wells), Jesse L. Martin (Joe West/Joseph West); Guest Starring: Keiynan Lonsdale (Wally West), Robbie Amell (Ronnie Raymond/Deathstorm), Teddy Sears (Jay Garrick), Demore Barnes (Henry Hewitt), Violett Beane (Jesse Wells), Michael Rowe (Floyd Lawton); Written by Katherine Walczak; Directed by Millicent Shelton
After closing all but one of the breaches to Earth-2, Barry joins up with Wells and Cisco to use the last gateway to cross the barrier in an effort to save Wells' daughter, Jesse, from Zoom. As soon as they arrive they're stunned at the Earth-2 version of Central City, which is simultaneously retro and futuristic, with an elevated bullet train of sorts and vintage large prop planes filling the sky.
The cool factor has actually begun before that: when the trio crosses the barrier between earths, we get quick images of a number of things, including John Wesley Schipp (Barry's father on this show) in costume from the 1990s' TV series of The Flash, and Melissa Benoist as Supergirl (nicely foreshadowing the March crossover between her and Grant Gustin's Flash).
While Wells is all business, Barry and Cisco are fascinated in exploring different aspects of this world, particularly the dopplegangers of them and their friends. Iris is a detective and Barry's wife, Joe is an embittered singer who hates Barry; Mr. Allen – Bartholomew here – isn't too fond of Joe either, and much more of a nebbish (think Marty, Jr. in Back To The Future Part II and you'll get a sense of what they're going for, right down to our Barry taking that one's place after he's been rendered unconscious); Floyd Lawton, who was Deadshot on Arrow but as a cop can't hit the broad side of a barn when he shoots here; the late Ronnie Raymond, better known as Deathstorm (rather than Firestorm),who has cruelly kept the Martin Stein part of himself locked away within him ("He doesn't speak that much anymore," Ronnie wryly notes); Caitlin Snow who has become the villainous meta-human Killer Frost; and Cisco's evil double, Reverb, who has mastered his powers in a way that Cisco didn't even realize was possible.
Despite Wells' arguments to the contrary, Barry can't help but feel that many of these people are his family, and he insists on helping them - especially after they've been threatened in various ways by Killer Frost, Deathstorm, Reverb and, ultimately, Zoom, with Joe being killed as a result. In the end, this bites all of them in the ass when, after Flash is being assaulted by Deathstorm and Reverb, Zoom arrives, kills those two for disobeying orders, captures Flash and locks him in a cell across from Jesse, stating this will be the last place he will ever see.
Things aren't exactly going so well on our world: a villain named Geomancer shows up, able to unleash shockwaves and emboldened by his belief that The Flash is no longer around; Jay Garrick admits that Wells didn't take away his speed force on Earth-2, that his desire for more speed is what led to his ingesting Wells' "Velocity 6" formula that drained his power – and things don't exactly work out when he temporarily gains his speed back to briefly go up against Geomancer after drinking "Velocity 7." Worst of all: the gateway to Earth-2 has collapsed and if Caitlin and Jay can't get Barry and Cisco back within 24 hours, they will become permanent residents of that alternate world.
The Flash remains the series in the superhero sweepstakes that most embraces its comic book roots; delighting in those tropes and digging as deep as it can into DC's mythology. Praise must go to production designer Tyler Bishop Harron and the visual effects team for so successfully painting an image of a world that is similar to yet different from our own. Also, the script for this episode works on virtually every level, and the performances across the board are strong, anchored as always by Grant Gustin's portrayal of Barry Allen. He continues to bring to his character a fanboy's excitement over everything he's experiencing; he's earnest without being cloying and when it comes to the deeper emotions, look no further than the moment when Barry is talking to his still-living Earth-2 mother on the phone. The way he barely holds himself together during this conversation – joy coupled with a personal sense of loss and an awareness that this is a gift he'll never experience again – is simply wonderful to watch.
The Flash may hit an occasional speed bump (sorry) along the way, but it pretty consistently delivers the goods. Welcome To Earth-2 is a perfect example.