While the Winchesters worked to seal off the Gates of Hell through the Three Demon Trials, the angel Castiel was approached by fellow angel and the Scribe of God Metatron to do the three Angel Trials to seal all of the angels in Heaven and force them to sort out their differences. Castiel and Metatron were able to kill a Nephilim and start the process to steal a Cupid's Bow, but Metatron got captured partway through the process. With the help of Dean Winchester, Castiel was able to get the bow, but the two were approached by Naomi who warned them that the whole thing was a trick and Metatron was really plotting to expel all the angels from Heaven onto the Earth. Castiel refused to believe Naomi due to her previous lies, but going to Heaven to confront her, he found Naomi dead and Metatron admitting she was telling the truth. Metatron ended up stealing Castiel's grace to use as the last ingredient in his spell and sent Castiel to Earth to live a human life before making all of the angels fall.
After becoming human, Castiel arrives in the forest near Longmont, Colorado where he is nearly hit by a truck and is surprised to find he hurt his hand. While the truck owner doesn't have a phone, he drives Castiel to a nearby gas station to use a pay phone and gives him some money for the pay phone and to get some food if the gas station has any. Castiel reluctantly takes the money and walks to the phone which a biker is using. After the biker refuses to hang up the phone, Castiel tries to stun and then smite him, forgetting he has no powers. The unamused biker threatens to stab Castiel once he is done with his phone call so Castiel walks away.
As Castiel walks away from the phone, he is approached by a young woman who claims to recognize him. While Castiel tells her he doesn't know her, the young woman tells him he is Castiel, that they met in Heaven and that her name is Hael. Castiel realizes that Hael is another angel which she confirms, but asks him if she's really an angel without her wings. The two sit down and Hael describes the Fall from her perspective, wondering how it could've happened. Castiel lies and tells her that he doesn't know and confirms that his grace is gone, though he can still hear angel radio. Hael tells Castiel that the angels are afraid and that many are still looking for vessels for themselves. Castiel tells Hael that there's nothing to be afraid of as while there was order and purpose in Heaven, on Earth the angels have the freedom to do whatever they personally want, not what they are told to do. When Hael wonders what she should do, Castiel asks her for suggestions. Hael tells Castiel that the last time she was on Earth, she created the Grand Canyon and would like to see it again. Castiel decides that they will go to the Grand Canyon and see it together.
Before they leave, Castiel is able to call Dean and tell him that Metatron tricked him and made the angels fall. Dean tells Castiel that Sam is dying and asks if Castiel has heard his prayers. Castiel tells Dean that Metatron took his grace and for Dean not to worry about him. Castiel asks what Dean is doing to save Sam and Dean tells him about "Ezekiel" trying to heal Sam. Castiel tells him that Ezekiel is a good soldier and will be able to help him until he can get there. However, Dean tells Castiel not to come as there are angels after him and the angels are pissed. Castiel tells Dean that some angels are simply lost and looking for direction and that he has met one who he wants to help. Dean warns Castiel against this as helping angels is what got him in trouble in the first place and tells Castiel until they can get everything sorted out, to trust no one. When Castiel still insists on helping, Dean reminds him that there's now a war on between the angels and its Castiel's fault so they will be searching for him. Dean points out that without his grace, Castiel is human and will now have to deal with all the human things that come with it. As Castiel tries to continue to argue, the hospital Dean is at starts to rumble and Dean realizes that another angel is likely coming. Dean orders Castiel to get to the Men of Letters bunker alone and hangs up to deal with the new angel arrival. Despite what Dean told him to do, Castiel decides to head to the hospital to help him. Hael tries to get Castiel to let her come with him, but Castiel refuses, telling her that its best if he goes alone and that this is her chance to help people. As Castiel tries to leave, Hael hits him over the head from behind with a two-by-four, knocking him out.
After knocking Castiel out, Hael takes him in her car and takes off. When Castiel wakes up, Hael tells him that she couldn't just let Castiel leave as she'd be lost without him. Hael explains that she feels this is the least he could do after causing the angels to fall and her plan is to go to the Grand Canyon with Castiel telling her more about humans along the way and her showing him what she is capable of. When Hael tells him that she intends for them to become one, Castiel realizes that Hael intends to possess him. Hael confirms this, telling Castiel that his vessel is strong while hers won't be able to hold her for much longer and is already deteriorating. Hael believes that possessing Castiel, she could learn to like the experience of living on Earth. As the two continue to drive, Castiel notices that Hael has an angel blade in her lap and is not wearing her seatbelt. Castiel puts on his own seatbelt and then purposefully crashes the car into some concrete blocks, sending Hael flying through the windshield, incapacitating her.
Waking in the car after being knocked out in the crash, Castiel goes outside where he picks up Hael's angel blade and finds the injured Hael on the ground. Castiel tells Hael that he doesn't want to hurt her or any of the other angels and will devote his life to helping the other angels adjust to life on Earth. Incredulous, Hael tells Castiel that the angels don't want his help, they want revenge on him and tells Castiel that if he allows her to possess him, she can protect him. Castiel refuses Hael's protection and goes to leave, but Hael threatens that if he leaves her in her broken vessel, she'll tell the angels where he is and they will hunt him relentlessly for revenge on the angel that destroyed Heaven. Castiel orders Hael to stop, but she continues on, telling Castiel that the angels will seek a vengeance upon him that will make God striking down Lucifer look like "child's play." Though Castiel continues to demand Hael stop, Hael continues on, telling Castiel that she will go on angel radio and tell the other angels everything if he doesn't say yes to her. With no other choice, Castiel stabs Hael in the chest with her own angel blade, killing her.
Following his killing of Hael, Castiel is forced to abandon his beloved trenchcoat that he has worn since he first took Jimmy Novak as his vessel due to a lack of money to pay to wash his clothes and buy food. Castiel eventually gets another trenchcoat after he steals Theo's grace to become an angel again.
Like Hael predicted, the angels chase after Castiel for revenge, but he eventually manages to elude them. As Castiel promised Hael, Castiel works to help the angels and eventually begins organizing them against Metatron to retake Heaven. With the help of the Winchesters, Gadreel and Hannah, Castiel eventually manages to defeat Metatron and retake Heaven, restoring his reputation, but is once again a pariah after letting Metatron escape.
- After this fight, Castiel discard the trenchcoat he has worn since his first appearance in the season 4 premier, five years earlier. He eventually gets a new one to replace it, but doesn't replace the tie until Claire Novak comments on how he looks better in one.
- Castiel is shown to struggle with being human, having to remember he has no powers anymore and needs to do things like eat and wear a seatbelt. Castiel shows a greater adaptation than the other angels, such as knowing he needs to wear a seatbelt when in a car.