Sam's research into artefacts powerful enough to stop Amara leads him to something called "the hand of God", an item found - and then lost - by the Nazis in World War Two.
It turns out that before the Nazis could do anything with the item it was stolen by a Men Of Letters' operative, Delphine Seydoux (Weronika Rosati), and then she and artefact were to be ferried back to the States by submarine. Only the sub was sunk and the "hand of god" lost.
Dean suggests that Castiel (who is actually Lucifer, but the Winchesters don't know this) send him back in time to liberate the artefact from the submarine just before it's sunk.
Lucifer - who has reclaimed his throne in Hell and relegated Crowley to his caged pet - thinks this is a great idea, seeing an opportunity to obtain a powerful relic. Unfortunately, he hadn't bargained on wards put in place by Ms Seydoux to keep supernatural entities away.
Dean then find himself trapped on the sub. Luckily he quickly convinces Delphine of his mission, although the submariners are less easily persuaded.
Back at the Bunker, Sam and "Castiel" race to find magic strong enough to dispel the wards and bring back Dean and "the hand of god". In the process, though, Lucifer lets his mask slip.
As a Cas fan, I was pleased that the "Lucifer-pretending-to-be-Castiel" ruse was drawn out through too many episodes and as this stage of Supernatural's history it could easily have outsayed its welcome.
As an episode The Vessel is an odd one, even Robert Berens' script acknowledges in the end that Dean is little more than an observer in the events that unfold on the sub.
In fact, if you think about it (and this is where the time travel migraines set in), it probably would have been better for their cause if Dean hadn't travelled back in time - given the artefact's "limitations" - and they'd found a different way to recover the "hand of god" from the sunken sub.
Don't forget, several seasons back, Crowley retrieved the First Blade from a similar hiding place at one point.
On the other hand, it was good to see that The Thule Society's magic (from Everybody Hates Hitler) hasn't been forgotten, but as I said before I find it strange that such powerful - and evil - magics aren't a bigger part of the Supernatural-verse.