When Castiel wakes up in a strange world married to Dean, he thinks it may be Chuck showing him a version of the future. As days pass he quickly learns he's trapped there with a version of Dean similar to the real one, but jarringly different at the same time. Once he escapes, Cas grapples with the consequences of keeping what he saw a secret or telling Dean the truth.
When one of his first hunts after becoming human again ends in injury, Castiel is faced with the helplessness of his mortality. Through it all Dean is a constant presence at his side, ever the attentive caregiver, which only serves to increase the tension growing between them.