SPOILER WARNING: This article contains major spoilers for "Starling City," the Arrow Season 8 premiere.
When Arrow Season 7 concluded, Oliver Queen joined the Monitor on his quest to warn other worlds about "Crisis on Infinite Earths." In "Starling City," the Season 8 premiere, Oliver made good on his promise. The Monitor sent him to Earth-2, where he sought Dwarfstar particles for an unknown purpose.
Fortunately, these Dwarfstar particles were in possession of Queen Industries. As a Queen heir, Oliver had no problem locating them. However, when he arrived at the lab as the Green Arrow, he found the particles stolen and the scientists dead. The culprit? The Dark Archer. As such, Oliver went looking for Malcolm to confront him, only to find the Monitor standing in Malcolm's office.
"It never ceases to impress: humanity's capacity for invention. Infinite number of worlds, and not a single one where man fails to emerge from caves and hold dominion over the Earth," the Monitor mused.
"What are you doing here?" Oliver demanded.
"Reminding you what you are doing here. This does not include interfering in the events of this Earth," the Monitor pointed out.
"I have no intention to interfere," Oliver replied.
"Are you not here to confront Malcolm Merlyn with your suspicion that he is the Dark Archer?" the Monitor inquired.
"Well, I went to applied sciences last night to retrieve the Dwarfstar particles, just like you asked me to," Oliver said, anger in his voice.
"You make it sound like a trifle," the Monitor scoffed. "Those specific particles can only be found on this Earth out of a multitude."
"My point is that, when I arrived, Malcolm had already stolen them. You know what might help? If I actually knew why I was after them," Oliver spat back.
"That I have tasked you should be reason enough, or have you come to doubt your commitment to this mission?" the Monitor asked.
"No, I have not," Oliver said. He continued on his mission, only to discover Tommy Merlyn was the Dark Archer after all. He had taken a villainous turn when his sister Thea died of an overdose in the Glades. He plotted revenge against the Glades, in the form of a new Undertaking. He planned to use the Dwarfstar particles to blow up the Glades. Oliver was able to talk him down and ultimately secured the particles for the Monitor.
The Monitor was likely in a hurry to retrieve the particles before Earth-2 was destroyed. As the episode drew to a close, Earth-2 came under attack. The universe dissolved around them, taking everything with it. Fortunately, Oliver managed to complete his mission before this happened. At the time of writing, the Monitor has not yet revealed why he needs these particles.
Airing Tuesdays at 9 p.m. ET/PT on The CW, Arrow stars Stephen Amell as Oliver Queen, David Ramsey as John Diggle, Rick Gonzalez as Wild Dog, Juliana Harkavy as Black Canary, Katie Cassidy as Black Siren, Katherine McNamara as Mia Smoak, Joseph David-Jones as Connor Hawke and Ben Lewis as William Clayton-Queen.