Previously on Arrow:
John Diggle was recovering from the tremor that he got when Adrian Chase blew Lian Yu island. Oliver Queen was back as temporary Green Arrow till John recovers and helped his team to fight against the criminal who is a kind of cyber mastermind working with Laurel Lance (Black Siren). But then FBI agent Samandra Watson who was investigating Mayor Oliver Queen (Star City) arrests him for being the Green Arrow. Oliver was then released on bail by the court and the proceeding will continue after the crossover.
Crisis on Earth X: Part two continues on Arrow
At the end of the first part Sara Lance (White Canary) and Alex (Supergirl sister) defeated Prometheus who once was one of the most dangerous villain of the Arrowverse on present earth. They bought Prometheus to the STAR labs, trapped him in a cell and Oliver finds out that the Prometheus was not Adrian Chase but Tommy Merlyn.
The teams (since all superhero team have combined) find out that the Nazis that spoiled Barry and Iris wedding were not from the past which means it was not a time anarchism (Basically breaking time and changing the history). Then the genius inventor of the STAR Labs Prof. Harrison Wells explains that this Nazis are from another earth, to be exact according to him they were from earth 53 also called as Earth X.
On earth 53 Nazis won the war and Hitler continued his invasion capturing the world until he died in 1994. As per Tommy Merlyn from earth X there was no place for emotions, sentiments and there were no schools, each of them were trained to be a part of Military to serve the Fatherland. Tommy killed himself by taking some dangerous substance like a cyanide or something refusing to provide information on why the Nazis were here.
Meanwhile Curtis another vigilante from team Arrow know by the name “Mister Terrific” along with Felicity are trying to find the location of the Nazis. Best part is when the Oliver (Green Arrow) meets Oliver (Red Arrow because he was Nazi) from earth X, Kara (Supergirl) meets the evil Kara from earth X and Flash meets Eobard Thawne of earth X.
Barry hates Thawne because on this earth he killed his mother. Another interesting concept that DC TV introduced was the love chemistry of Kara (Evil Supergirl) and Oliver Queen (Evil Arrow) and they both were married. Although truly speaking this is one was the horrible superhero couple because in real comic books they hardly knew each other.
The Evil superhero team steals a prism and luckily Green Arrow gets a shot on Evil Supergirl with kryptonite arrow. The teams at STAR Labs use the blood sample to trace the location of the Nazis hiding and also the prism which they stole.
The Elite Superheroes fail fighting against Evil Supergirl and Thwane along with the Nazis army including Metallo. Metallo is a kind of Justice League Cyborg with power of kryptonite that can be used against the Supergirl or her cousin Superman. While at the same time Evil Arrow secures the control over the STAR Labs defeating Killer Frost (Catlin Snow), Heat Wave (Mick Rory), Mister Terrific (Curtis) and Black Canary (Dinah Drake another vigilante from team arrow).
This all invasion purpose was revealed in this end of this episode. The Evil Supergirl was getting weak because she absorbed too much of sun energy. She basically needed a heart transplant and they found Kara (Supergirl) who will be the donor. After a little chit-chat Metallo knocks of the Ellite team of superhero.
Oliver, Sara, Alex, Firestrom, Flash found themselves kept in a detention area and that place was on Earth X. Luckily two ladies Iris and Felicity have survived from being captured and are trapped inside STAR Labs.