Cas - now a human - is living as Steve, a sales assistant at a petrol station, when he learns of recent strange missing persons' cases and alerts the Winchesters.
Dean opts to take this case solo, leaving Sam and Kevin back at The Bunker working on a translation of the "angel tablet" to try and find a way of reversing the spell that cast all the angels out of Heaven.
Investigating the disappearances, Dean discovers that the victims have actually been reduced to a fine, pink, mist.
When Cas joins Dean on the job the ex-angel recognises the handiwork of an angelic healer (a Rit Zien) who has been drawn to, and overwhelmed by the new sensation of, the pain and suffering of humanity.
It's always great to see Misha Collins front and centre as Cas, especially when - as in Heaven Can't Wait - he is awkwardly learning about the human condition.
Although the sub-plot of Sam and Kevin having to deal with Crowley, in the hope that he can finish the translation that Kevin is stuck on, is crucial to the overarching story of the season, this episode is really the Cas and Dean show.
Ephraim (Ashton Holmes), the Rit Zien, makes for an interesting 'villain' as, to all intents and purposes, he believes he is doing good... it just involves vaporising people who are in mental pain.