As you can probably tell from the title, it's a crossover episode with the cast of 'Arrow'. I've never watched the latter, but he/they certainly work well with Team Flash. Flash/Barry and Arrow/Oliver make for a nice odd couple, their contrasting personalities and styles making for some interesting drama. And when the freak of the week infects Barry with something that causes him to go into an uncontrollable rage, it's on as the two costumed heroes square off (with far less collateral damage than their big screen DC brothers).
The freak of the week is potentially interesting, but he's too underdeveloped, having to share time with not only the Arrow stuff, but a rather unnecessary side plot involving the police station being taken hostage. The highlight is obviously the Flash/Arrow confrontation, but after Barry is cured he and Arrow make nice and vow to go get the freak of the week... which we don't get to see. The next scene is literally them walking into S.T.A.R. labs and putting him in a cell. Huh? But despite the misuse of the villain and the excess side plot, the Arrow/Flash stuff is strong enough to carry this episode over the top.