This is the first episode of Arrow to have scenes on an alternate earth.
This episode feature many references to past episodes of the show, mainly from Season 1: Throughout the episode, Oliver experiences events in extremely similar ways he experienced the same events on Earth-1 or in Pilot (2012), but with a few changes, notably Malcolm Merlyn taking Walter's role as well as Tommy Merlyn taking Thea's role. Malcolm mentions a fight between the Hood and the Dark Archer to Dinah in a similar way Oliver told Quentin about the Hood saving him in Pilot (2012). Oliver tries to escape Diggle's car but Diggle beats him to the chase, revealing he is the Earth-1 version. He also tells Oliver that he fooled him once, which also refers to Pilot (2012). After Oliver leaves a party at Verdant, Diggle points out he's going the wrong way. The same thing happened in Pilot (2012). Also, Oliver ends up stopping Diggle the same way he stopped him in that episode, even pointing out that he always falls for that move. Oliver and Tommy mentions that Thea used to say "The dead don't want anything. It's one of the benefits of being dead." Earth-1 Thea said the same thing in Honor Thy Father (2012), and Oliver referenced that quote to her in Legacy (2016). Moira mentioned that Thea died from a Vertigo overdose on her 18th birthday. This is a reference to Trust But Verify (2013), where Thea drove after taking Vertigo on her 18th birthday, which resulted in her going to court. The fight between Oliver (as the Green Arrow) and Tommy (as the Dark Archer) is similar to the fight between Oliver and Malcolm in Darkness on the Edge of Town (2013). When Oliver finds out Tommy is the Dark Archer, he reacts the same way Malcolm did in that episode when he found out Oliver is the Hood. Oliver gets trapped by Tommy the same way he got trapped by Malcolm in Sacrifice (2013). He later get out of the situation the same way he did in that episode, while Diggle saves him and jokes that he found him from a tracker in his shoe (which also happened in that episode). The final fight between Oliver and Tommy on a roof is extremely similar to the final fight between Oliver and Malcolm in Sacrifice (2013). Oliver says to Adrian that he's 10 steps ahead of him, the same thing Earth-1 Adrian used to tell Oliver in Season 5. Oliver meeting a tech expert for Queen-Merlyn Enterprises echoes when he met Felicity during Lone Gunmen (2012).
Bruce Wayne's quote shared by Adrian Chase, "once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth" is taken from Sherlock Holmes; appropriately both are the Great Detective.
The city where Oliver and company reside on Earth-1 has not been called Starling City since Green Arrow (2015) when it was renamed Star City as a method of rebranding after the repeated tragedies and in honor of Ray Palmer who was believed dead. Oliver changed his own vigilante name to the Green Arrow that same episode. This is the Earth-2 version of Star City which never underwent the name change, most likely because Oliver never returned and set the events in motion that led to the change.
John notes how Adrian is acting like Oliver, Curtis and Felicity all rolled into one. Additionally the other characters also take roles of their Earth-1 counterparts: Malcolm Walter's, Tommy Malcolm's, Dinah Quentin's, and Thea Rebecca Merlyn's (as the catalyst for the Dark Archer's Start of Darkness).