The common wisdom is that if you live long enough, you'll see everything.
I suspect that was penned long before the internet, but no matter the principle is the same.
With so much wonderful content floating around the cloud, the job of the viewer is to find the best, and the job of the reviewer is to identify the best.
One trick to this is that the core arc, no matter how clever, is never enough to push a show over the top. The characters have to be interesting and moreover they need to appear to be having fun so that, by proxy, the viewer can have fun too.
(Digression -- this was the "big secret" in Buffy. Not only were the Scooby gang fighting Evil, but they were having fun at the same time. The irony is that the one single person on the planet who should understand this better than anyone - Joss -- has overweighted much of SHIELD with tedious arcs while the characters themselves seem to be having a rotten time. And now back to our review...)
This wonderful episode makes the point. On the one hand it is about saving the world, yada yada. But everyone is having a great time en route and gets to spout great dialog.
Best of all, my favorite character, Bridget Regan as Dottie, is back. Even in a show that is already upbeat, she steals all her scenes and, armed with fast moves and great lines, takes the whole show up several notches in the blink of an eye.
It does not get any better than this.