In order to defeat an alien invasion, the Flash is going to need a little help from his friends. DC All Access' exclusive clip from tonight's episode of "The Flash" shows Barry zipping over to Star City to recruit Oliver and the rest of Team Arrow to his cause. Check out the clip starting at the 2:30 mark in the video below.

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Of course, Oliver and John "Spartan" Diggle are a little preoccupied when the Flash shows up. In the clip, both Green Arrow and Spartan are mid-mission, tracking down a violent new vigilante known appropriately as Vigilante. This isn't Oliver's first encounter with Vigilante; last time, Vigilante slipped through Team Arrow's fingers even as Green Arrow demanded that he stop bringing more violence to the streets. From their latest encounter, it seems like their relationship hasn't improved much, as Barry rescues Oliver and Dig from a spray of Vigilante's bullets.

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Barry then brings the two back to Felicity's apartment, where she had been running the op. It doesn't take Barry long to get into why he stopped by: aliens. He quickly explains that this threat is the biggest they've ever faced and that he's going to need everyone he can get to help stop them. When he mentions that Lyla also knew about the Dominator invasion, Dig became visibly upset; interestingly, it appears as though she didn't loop him into the whole "alien invasion" news, so it seems likely they'll be having a chat about that sometime soon.

The news of an alien invasion sparks some interest from Thea, who had stopped by on mayoral business. In fact, it appealed to her enough that she was willing to sign up for the fight, which means she'll become Speedy again despite her retirement in the "Arrow" Season 4 finale. This won't come as much of a surprise to anyone who has seen recent promos for the crossover, as she has featured prominently in several TV spots.

Once in the “Arrow”-verse, Supergirl will join the Flash, Green Arrow, the Legends of Tomorrow and their friends in order to stop an alien invasion. With them, she’ll face down the Dominators, mind-controlling aliens who have grown so concerned about Earth’s metahuman threat that they have decided to stomp it out before it can reach the stars.

Starring Melissa Benoist as the Girl of Steel, “Supergirl” airs Mondays at 8 pm ET/PT on The CW. The network is also home to Grant Gustin as the Scarlet Speedster in “The Flash,” airing Tuesdays at 8 pm ET/PT, Stephen Amell as the Emerald Archer in “Arrow,” airing Wednesdays at 8 pm ET/PT and Arthur Darvill, Brandon Routh and Caity Lotz in “DC’s Legends of Tomorrow,” airing Thursdays at 8 pm ET/PT.