The CW network has released a video teaser for the two-episode Flash vs. Arrow crossover that will air in December.  On the December 2nd episode of The Flash, Team Arrow heads to Central City to investigate what would appear to be a mysterious boomerang that is the weapon of choice for Digger Harkness (aka Captain Boomerang).  Barry Allen is excited that Oliver, Felicity and Diggle are visiting, and he asks Oliver to help him stop a meta-human with the power that makes people lose control of their emotions.
 
When things don’t go well, and Oliver criticizes Barry’s efforts, The Flash decides to proceed on his own with disastrous consequences as the meta-human infects Barry causing The Flash to go on a rampage and setting up the Flash vs. Arrow dynamic.