This review WILL CONTAIN SPOILERS for the episode to be discussed, if you do not want certain plot points or story spoilt, please do not read further.
After a few adventure of the week episodes this week's episode of Supernatural gets back to the main story of the Winchesters trying to find a way of fighting Amara.
When Sam finds references to items on earth touched by God, called the Hand of God, that possess massive destructive energy he and Dean believe that they may finally have found a weapon that they might be able to use against Amara, after all, if these items contain some of Gods power then surely they can hurt a being as powerful as him.
Digging into the history of the Men of Letters they discover that the group at one point possessed one of these items, when a female agent of theirs (a Woman of Letters) called Delphine stole it from the Nazi's during World War 2. Sadly the submarine that was transporting Delphine and the Hand of God to America was attacked by a German warship and sunk with all hands lost.
With no other leads to finding other Hands of God Dean comes up with a drastic plan, to get Castiel to transport him back in time onto the sub to retrieve the weapon before it gets sunk. Lucifer (still secretly possessing Castiel) agrees to the plan, knowing that he alone doesn't have the power to defeat Amara, and sends Dean back.
Unfortunately Delphine has warded the sub against supernatural creatures, that include angels, leaving Dean trapped alone with no way back to the present.
Dean manages to find Delphine and convince her that he's from the future and needs the hand of God to defeat Amara, getting her to agree to destroy the warding on the vessel. Meanwhile in the present Sam finds a spell that will allow Lucifer to break through the warding, but gives up hope of it working when he realises that it will need the power of an archangel.
Lucifer tells Sam that he will 'try' the spell, still pretending that he's Cas, but when Sam suggests that he gets extra power from touching Sam's soul Lucifer can't keep a straight face any more and reveals his true identity. Lucifer tries to kill Sam but is stopped briefly by Castiel, who still has a small amount of control.
Back in the 40's the submarine comes under attack from the German warship, forcing the crew into desperate action. Knowing that they are going to be destroyed they decide to at least take their enemy with them and Delphine powers up with the Hand of God moments before Lucifer jumps in and snatches Dean and the Hand of God back to present day.
Sam warns Dean as Lucifer attacks the two of them, attempting to gain possession of the Hand of God. Unfortunately the object has been completely depowered and appears to be a one shot weapon. Lucifer contents himself with being able to at least kill the Winchesters, but gets cast out of the bunker by a blood ward created by Sam.
Supernaturals time travel episodes are always enjoyable, mainly due to Dean's inability to blend into any other time period than his own, but the results of Dean in World War 2 are definitely much more sobering than him trying to be a cowboy. For the first time in a long while (since the episode 'The End') that didn't really have a lot of laughs, instead choosing to make Dean watch the deaths of everyone around him,
The conversation he has with one of the crewmen asks Dean about the future of the war to see if his friends and family have a better chance of surviving is a small moment, but one that manages to pack in a lot of emotion.
The reveal of Lucifer being in Castiel's body is a welcome one, as it keeps the momentum of the series going, and adds extra conflict for the Winchesters as, despite saying otherwise when trying to get Sam to agree to be his vessel) Lucifer seems to have in no way changed his ways and wants revenge on the brothers for stopping him back in season 5.
The episode also gives us a brief look at what has happened to Crowley now that the real King of Hell is back on the throne. Crowley, surprisingly, has been kept alive, and seems to be some kind of pet for Lucifer, kept chained up in a hole in the wall in an awful shirt. Keeping Crowley around is definitely a mistake on Lucifer's part, and I'm sure he'll manage to break free at some point and rush to aid the Winchesters in getting him out of Castiel and back inside the cage,
'The Vessel' might not be the best time travel episode the show has done, but it introduces a new element the the lore of the Supernatural universe and keeps events moving at a brisk pace with the reveal of Lucifer inside of Castiel.