In about a month, the Arrowverse on CW Network will unveil its latest crossover special, Elseworlds (not to be confused with my own Arrowverse Elseworld — yes, blatant plug). Greg Berlanti’s production company and his creative staff have rolled out the crossover in the tradition of crossing over characters in the comic books they’re based on.

I’ve previously discussed the Crisis on Earth-X crossover and why the DC TV Universe has worked better in building up excitement for heroes coming together than DC’s attempt to build a shared cinematic universe.

The tradition of building to crossovers arguably started when Arrow was the only DC show featured on CW. During Arrow’s second season, the creative team rolled out a pair of episodes featuring Barry Allen, who would go on to get his own TV series, The Flash.

The two episodes, “The Scientist” and “Three Ghosts,” moved along Arrow’s overarching storyline for its second season, while introducing viewers to somebody who Berlanti wanted to have featured in his own series. The rest, as they say, is history.

In the two episodes, Barry Allen plays a key role in helping Oliver Queen, John Diggle and Felicity Smoak determine what happened during a break-in at Queen Consolidated. Though Allen has yet to become The Flash, it’s the first time Allen teams up with Queen to solve a case. Along the way, Allen becomes integral in saving Queen’s life.

The episodes introduce audiences to Cyrus Gold — in the comics, Gold would become the zombie supervillain known as Solomon Grundy. In Arrow, Gold is re-imagined as a criminal on whom Brother Blood tests the super soldier serum called mirakura. The test is a success and Gold assists Blood in acquiring technology to mass produce the mirakura.

Though Queen ultimately foils Blood’s plans, we learn toward the end of The Three Ghosts that Blood is being backed by Slade Wilson, a former friend of Queen’s. The tale told through flashbacks shows Queen and Wilson forming a bond as they try to escape Lian Yu, only for Queen to betray Wilson’s trust. Wilson is injected with mirakura, which causes him to become unhinged. Thanks to Queen’s betrayal, Wilson become obsessed with bringing Queen down.

Not only do we advance the overarching storyline of Wilson’s pursuit of Queen, the stage is set for Allen to become The Flash, thanks to a final scene in the Three Ghosts episode. What’s also set is Queen taking another step toward completing his ultimate look as the Green Arrow — a mask Allen creates and presents to Queen as a gift.

Though technically not considered a crossover, the two episodes set up the idea for how shows would cross over with one another and how heroes would come together to solve a problem.

And like most of Season Two of Arrow, the episodes still hold up. Arrow’s second season remains the show’s best overall season and the appearance of Barry Allen for two episodes is one of many reasons the second season works so well.

In the coming weeks, I’ll look at the actual crossovers that have happened, as we build toward the Elseworlds crossover for Arrowverse this year. I’ve already reviewed Crisis on Earth-X, so I won’t go into that one again, but I’ll look at the others and see how well they hold up.