In “The Vessel,” Supernatural revisits the trope of time travel once again. We agree with Sam that time travel is a dangerous business, with the risks often outweighing the benefits. We love time travel in Doctor Who, but it’s never been a great fit on Supernatural, with the exception of “The End” (S05E04). Yet writer Robert Berens (“Girls, Girls, Girls,” “The Executioner’s Song,” and “Our Little World”) and director John Badham (“The Hunter Games,” “The Things They Carried,” and “Into the Mystic”) seem to make it work in “The Vessel.”

Sam (Jared Padalecki), the tireless researcher that he is, has uncovered a possible weapon against the Darkness in the Men of Letters records. Is there anything left in that library he hasn’t read? During World War II, the Nazis uncovered an artifact known as The Hand of God, which was a piece of the Ark of the Covenant. The object was stolen by Delphine Seydoux (Weronika Rosati), a French Woman of Letters (!), and then disappeared when the USS Bluefin went down in the Atlantic in 1943. Dean (Jensen Ackles) goes full-on Raider by going back in time to try to rescue the Hand of God from its watery grave. This time-travel feat can only be accomplished with the help of Castiel, who is eventually revealed to the Winchesters as Lucifer.

The episode could have been a dud if it hadn’t been for the always entertaining Casifer and the committed sailors of the USS Bluefin. Lucifer working alongside Sam, while pretending to be Castiel, was fun to watch, but it was Dean’s involvement with the crew of a doomed sub that grounded the episode. Dean was never mocking in his treatment of the people he knew would die. And the sailors remained true to a mission that they came to know would end their lives. Delphine and the sub officers were ready to die for a cause they believed in, with one of the sailors asking Dean when the allies will win the war, thinking of all his friends and family still fighting. When Dean returns to the present, he’s clearly affected by events. He tells Sam, “I was just a witness.”

 Misha Collins as Lucifer remains impressive to watch. Here we saw more of Satan’s mood swings and less of his pretense as Castiel. Could Collins have predicted that his throaty choice for Castiel’s voice back in Season 4 would help him to differentiate Castiel from Casifer so effectively in Season 11? But of course, it’s much more than that—the looser body language, different speech patterns, the smirking, the laughing. When he comes down the bunker stairs covered in seawater, he’s practically unrecognizable as Cas.

It seemed a bit unbelievable that Sam wouldn’t recognize his old possessor and cellmate Lucifer, or at least intuitively realize something was wrong. He did just see Lucifer, so the idea that the Devil could find a way out of the Cage should’ve at least been a consideration. It was fascinating to watch Casifer observe the brothers and their interactions with each other. Will Lucifer learn something of humanity from them? For now, Lucifer is ready and willing to kill Sam, even if Sam is like the girl who kept turning him down at the prom.

We also enjoyed watching an aloha-shirted Crowley playing the part of Lucifer’s bad dog. Mark Sheppard manages to convey a whole lot of meaning with only a few lines of dialogue. Crowley has realized that Lucifer isn’t yet strong enough to defeat Amara. The Devil will eventually come to regret keeping this tricky pet around.

They lose the weapon (unless of course Delphine manages to survive somehow), yet the episode never feels futile. Certainly learning that Lucifer is lurking in Castiel’s vessel moves the story along in a significant way, but the story of the USS Bluefin reminds us that you can lose a battle, but still win a war.

The Supernatural mythology runs deep after 11 seasons. “The Vessel” connects to angel lore, the Men of Letters, and reaches back to the Season 8 episode “Everybody Hates Hitler” by bringing back The Thule Society. The layered richness of the series helps keep episodes such as this from feeling too thin.

“The Vessel” appeared a weak offering housed within such a powerhouse Supernatural season, but it had its moments. After only getting Demon Dean for three episodes in Season 10, we hope to see more of the fantastic Casifer in Season 11. We can’t wait to see what the Devil gets up to next.