So it begins with Felicity being approached at work by Cisco and Kate, both of whom are eager to see the Arrowcave and all the gear as well as help with the DNA test on the arrows that killed Canary.
Felicity takes them there, and Oliver is a bit uptight about it at first. Cisco of course loves everything he sees and mentions some ideas for an upgrade for Oliver.
Captain Boomerang, named of course by Cisco, is after Diggle's girl as she was the one that gave the order to kill the Suicide Squad that Boomerang was a part of, but his explosive didn't work and he is out for payback.
He attacks an Argus base and Arrow/Arsenal arrive, and soon receive and assist from The Flash.
Oliver is reluctant to team up and reminds Barry that Starling City is a lot rougher then Central City. Of course they argue and bicker and hurt each other's feelings a bit then reconcile and team up.
Diggle's girl sees Barry's face minus the mask......seriously
they need to stop that. She also gets a boomerang in the shoulder and Barry zips her to the hospital after Kate stabilizes her.
After that, they locate Cap Bommerang at the train station, Flash zips all the bystanders out of the way and they confront him.
Turns out he has a failsafe, 5 bombs in 5 locations, 90 second countdown and they are rigged where if one is disarmed the others receive the detonation signal. Barry finds them, but dares not move the bombs to safe spots, so he zips back and forth from the Arrowcave taking Arsenal, Felicity, Cisco and Kate one by one to each bomb. Barry takes the 5th bomb and they all cut the wire on each one at the same time. Please note these bombs had may bricks of C4 so running them out of the city would NOT have been the best if plans.
Arrow takes down Captain Boomerang and they lock him up with Slade Wilson on the submarine. (note: slade wilson is slated to appear in Arrow season 3)
Kate takes DNA samples from the arrows to try to determine the killer of Canary. Cisco gives Oliver a uniform that is kevlar weaved, and lighter then his current one thus he can carry more gear.
Oliver has a case setup for Barry to store his costume when he returns.
The two of them then go to an empty warehouse to see who can beat whom in a fight.
As to who wins, they don't show that.
Logically, Barry should win as he and throw many punches before Oliver and throw one, however Oliver has tactics and skills that Barry doesn't.
So the door is open for more crossovers. Too bad Gotham isn't in the same universe or timeline, so we can't expect Batman to show up.