'Arrow' Has a Surprise 'The Flash' Crossover in "The Devil's Greatest Trick"

Tonight's episode of Arrow saw the against Cayden James (Michael Emerson) and his explosive plan for Star City come to a head, but Team Arrow had a little extra help -- from the Flash!

Spoilers for tonight's episode of Arrow, "The Devil's Greatest Trick", below.

In tonight's episode, the clock was ticking on Cayden's plan to blow up Star City all because of that pesky video showing that Green Arrow/Oliver Queen (Stephen Amell) was responsible for the death of his son. However, because Oliver wasn't even in Star City when the murder took place, the team knew it was fabricated. They finally managed to figure out how to reveal the real video that showed the real killer was a paid assassin. Despite being warned that Cayden might be beyond reasoning with, Oliver decided to try anyway -- even with time at a premium.

But who do you call for an assist when you need to get somewhere fast? The Flash, of course! That's right, to help get himself in position and Diggle/Spartan (John Ramsey), Green Arrow called in a favor from his friend Barry Allen/The Flash (Grant Gustin). While we don't see or hear The Flash, we do see his super speed in the form of his signature lightning as he rushes the heroes to where Cayden is. Little mini crossovers like that aren't uncommon in the Arrowverse, particularly with The Flash and Arrow. Last season during the episode "Second Chances", Oliver, as Green Arrow, had to call on Central City Police Captain David Singh with The Flash needing to vouch for him in the process.

The Flash helping his friends out in tonight's episode also marks Barry's return to heroics. For those who haven't been following along on The Flash, Barry was wrongly convicted of murder during the midseason premiere of that series and has been doing time in Iron Heights Prison -- the same prison even down the cell where his father served time after being wrongly convicted. However, thanks to some unusual metahuman powers possessed by the newest member of Team Flash, Barry was released from prison and cleared of all charges. While this week's episode of The Flash ended with Barry and his loved ones celebrating his freedom with a casual gathering at home and not doing serious crimefighting in Central City, considering Barry's history of being there for all of his friends when they need him, it makes sense that Barry would suit up to help Green Arrow try to stop Cayden James.

Why Barry didn't stick around to try to save Star City? Well, the Flash is pretty busy trying to deal with The Thinker back in Central City. One thing for sure, though, Diggle had a major milestone when it comes to super speed tonight. This is the first time he hasn't gotten sick when Barry rushed him somewhere.

Arrow airs Thursday nights at 9/8c on The CW.