This week's episode had a couple stories going on at once (well, two sides of one story, actually) and neither was all that great. I might as well be honest about that now.
Sam discovers an artifact called The Hand of God may have the power to destroy Amara. They call upon Castiel (who, you'll remember, is possessed by Lucifer) to learn more about it, only to learn that the artifact sunk with the submarine being used to transport it...in 1943. See where this is going.
Lucifer, who lacks the power to stop Amara on his own, sees the opportunity to get the Hand of God and offers to travel Dean back in time to the submarine right before it sank. This is where the episode first started to lose me. I'm not a fan of the time travel episodes. I like time travel, but in this show I find it doesn't work
Dean and Lucifer make the jump, but special warding on the sub prevents Lucifer from getting on board, forcing him to return to the present to help Sam figure out how to get Dean back while Dean searches for the Hand of God. Meanwhile, the plot with Dean aboard the sub is tedious and boring as he searches for Delphine Seydoux, the Woman of Letters who has the Hand of God and lots of "I'm from the future, you need to trust me" talk ensues.
In the present, Sam finds a spell that will allow "Castiel" to remove the warding from the sub and offers up his soul to power it. Lucifer reveals himself to Sam and almost kills him, but Castiel regains control long enough to stop him and tell Sam what's going on.
Even the climax isn't very climactic. The entire thing is just rather dull all around. While the reveal to Sam and Dean of Lucifer being in Castiel's body is important to the overall story, the rest of the story around it isn't worth seeing.
Final Grade: D