When Cas arrives at the Bunker, clearly ill, and keeps having trouble remembering Dean or Sam or English, the brothers have to resort to a spell which paints Cas in sigils in an effort to anchor him to his own memories. The brothers aren't prepared to experience Cas' memories for themselves. Through events they thought they knew and ones they didn't knew existed, Sam and Dean will have to face the fact that with the years between them, and even with Dean secretly learning Enochian, they still haven't fully translated Castiel. If this works, they will have Cas with them and a whole new understanding, but the full extent of what it means to be Cas might be more than Dean can handle.