This review WILL CONTAIN SPOILERS for the episode to be discussed, if you do not want certain plot points or story spoilt for you, please do not read any further.
Using one of the most iconic Superman stories of all time for the Supergirl television show is a ballsy move, but luckily it's one that pays off well and actually makes one of the best episodes of the series yet.
Following straight on from the surprise ending of the last episode Kara's friends worry when she fails to appear for work the next day and won't respond to any calls and rush over to her apartment where they find her unconscious with a Black Mercy plant attached to her chest.
Comic book fans will instantly know what this means and the viewers are quickly let in on the details as Kara finds herself trapped in a dreamworld thanks to the parasitic plant. This is a good thing too, as it does away with the familiar television trope of trying to make the audience think that the hallucination could somehow be the real reality (such as 'Normal Again' in Buffy The Vampire Slayer, or the phenomenal 'Far Beyond The Stars' on Star Trek Deep Space Nine).
Whilst Alex, Winn and James fight to find a way of getting the Black Mercy off of her Kara finds herself back on Krypton with her family alive and well. It's the perfect world she could have wanted if her home world hadn't been destroyed, she even has a young Kal-El and an innocent and free Aunt Astra in her life.
My one complaint with the Black Mercy story is that the team devise a plot to free Kara from the Black Mercy by using a virtual reality device to send Alex into Kara's mind to get her out felt a little weird and kind of pointless. Why go to all of the trouble of trying to make the VR work, even going so far as to get Maxwell Lord to help with it, when J'onn could have just used his telepathy to send Alex into Kara's mind? It kind of feels like the writers overlooked that possibility, or at least failed to give a reason why J'onn didn't used his powers to solve the problem.
The Black Mercy story wasn't just a throw away story though, as it fed into the bigger season arc as we find out that it was Non who sent the plant to Kara in order to distract her and keep her out of the way whilst he and his men put parts of their plan into motion.
Using Non was a great idea because it gave Kara someone she could really taker her anger out on. Having just gone through the pain of losing her old home and her family for a second time she needed a foe that she could unleash some of her anger on, and it being a Kryptonian means that she doesn't have to pussyfoot around and can let loose. It's not quite up to the levels of violence she used on Red Tornado, but it's pretty close, plus it's got the emotional impact that the Red Tornado fight lacked.
It wasn't just Kara who got to fight though, as both J'onn and Alex took on Astra. Whilst it was great to see J'onn back to being the Martian Manhunter and using his powers to full effect (including phasing during a fight!) Alex dispatching Astra with a Kryptonite sword was definitely the highlight of the fight. It also raises some stakes for later in the season, as J'onn takes the blame for Astras death as to preserve her relationship with Alex.
This will surely be something that will rear it's head in the future, probably as a way of driving a wedge between Kara and Alex and create divisions amongst the DEA before the seasons climactic events.
Considering the original source material for this episode is widely considered one of the all time greatest Superman stories ever written Supergirl does a great job at adapting it to their own needs and capturing the essence of the original material whilst managing to make it their own. A good example of just how good the series can be and the kind of episodes they can make when at their best.