The episode begins with a meeting of the Scooby Gang at the Magic Box. Buffy bemoans that it's her birthday time again. Mom wants to make a big deal of it, but Buffy has bad luck with birthdays. Willow tries to encourage Buffy, who claims she's too busy with Glory to party. Xander is amazed that they're facing a god. Giles offers some of the information uncovered by the Watchers Council: Glory and two of her fellow hell-gods ruled over one of the rather nastier dimensions, until she found a way to our reality. She's immortal, invulnerable, and insane. She survives by extracting energy from the human brain. Once drained, all that's left behind is a crazy person. While the Gang discusses strategy, Xander wisely brings up the Key for which Glory has been looking. An uncomfortable Giles and Buffy try to dissuade them from worrying about it, but the Gang is insistent. Finally Buffy and Giles have to admit they know where the Key is. The Gang is disappointed that the two didn't let the rest of them in on it. Buffy agrees, "They're going to be risking their lives. They deserve to know." She tells them that Dawn is the Key.
The Knights of Byzantium are chanting around a bonfire: "The Key is the link; the link must be severed; such is the will of god." Jinx appears with some of his brethren and attack the knights. Just as Jinx is about to be killed, Glory steps in and fights the knights, killing most, and capturing one of them.
Tara and Willow are pouring magic sand around the Magic Box when Dawn walks up and asks them about the spell they are doing. The witches are setting up an early warning incantation. "If anything hell-goddishly powerful comes within a hundred feet of the shop, then screechy siren things will, you know, screech." They've already put one up around the Summers house. Dawn wants to help, but they all know Buffy wouldn't like it. Dawn gloomily enters the shop and starts teasing Xander, who, along with Anya, is uncomfortable around Dawn considering their new information about her origin. Giles and Buffy come out of the training room and talk a little with Dawn. During their conversation, Giles hides his journal in a secret drawer under the cash register. Dawn notices its disappearance, but not its new location. Dawn teases Buffy about fighting someone prettier than the Slayer. Buffy asserts that Glory is evil and powerful and in no way prettier than she is.
Glory is tormenting the Byzantium Knight she captured and now has tied up in her apartment. Despite the torture, he refuses to give her any information, so she sucks out his brain energy.
Although Buffy didn't want the birthday party Joyce is hosting at the Summers' home, she is nonetheless pleased about receiving the presents. Anya is excited at the suspense and wants the presents for herself. Buffy opens a pretty dress from Tara and Willow. Then she opens Dawn's gift: a frame comprised of seashells surrounding a picture of Buffy and Dawn at the beach in San Diego. It's a very sweet and sentimental gift that makes everyone think again of Dawn's supernatural origins. Buffy is impressed, and gives Dawn a thank you hug.
In the kitchen, Buffy, Giles, and Joyce are discussing Glory when Buffy notices Dawn listening in the dining room. She's just collecting plates; when she takes them in to the living room, the people in the room get quiet. Dawn gets angry at everyone for giving her the silent treatment and goes up to her room. Then she climbs down the trellis to exit the house. Spike approaches with a box of chocolates for Buffy, and he startles Dawn. He tries to scare her, but Dawn is not frightened. Indeed, she teases Spike about bringing a present for Buffy. She claims to be badder than he is: She's going to break into the magic shop to steal things. Spike points out the dangers between the house and the shop, so Dawn asks him to come along and steal stuff with her. He accepts.
Spike picks the lock, and they enter the shop. Dawn finds the secret drawer and Giles' journal. As she reads, Spike pokes about the shop and leaves a used cigarette in an urn. Dawn is confused about the part that reads, "those outside reality being able to see the Key's true nature." Spike explains that only those with second sight are able to see it, along with your run-of-the-mill lunatics. Dawn remembers the crazy people who have berated her. Spike sits next to her and reads from the journal that the monks sent the Key to the Slayer in the form of a sister to insure that the Slayer would protect it with her life. Spike annoyingly points out to Dawn that she is the Key.
Back at the birthday party, Willow, Tara, and Buffy are discussing the fact that Riley didn't even send a card due to the communications blackout. Tara then notices Dawn, who has slit her wrist with a knife. Dawn demands to know if she's real, if she's more than just a "key." Party's over.
After Buffy sees her guests to the door, she and Joyce talk with Dawn in her bedroom. Dawn wants to know why they didn't tell her. (They thought it would be better if they waited until she was older.) When did the monks create her? (Six months ago.) She's having an identity crisis. Even though the two elder Summers try to comfort her, Dawn kicks them out of her room.
The next day at the Magic Box, Buffy presses Giles for more information about the Key, not because of Glory, but because Dawn needs to know. Giles wonders how Dawn found his journals and got into the shop into the first place. Anya discovers the cigarette in the urn.
Spike is painting his fingernails when Buffy bursts into his crypt. He tries to banter, but she slams a concrete slab into him, demanding to know why he helped Dawn find out about being the Key. Does Spike hate Buffy that much? (If only she knew Spike's true feelings for her!) Spike points out that Dawn was going to the shop no matter what, and he thought she'd be safer with "big bad" looking over her shoulder. Dawn was going to find out anyway. Buffy couldn't keep it away from her forever. She's just kicking Spike around out of her own frustration.
Dawn is still in her bedroom when Joyce informs her that it's time to go to school. Dawn is still despondent and doesn't want to go. When Joyce coddles her daughter, Dawn becomes even more upset and decides to school to be away from Joyce.
At the hospital, Ben the intern is dispensing medicine in the mental ward when he sees that one of the patients is a Knight of Byzantium. Jinx appears, and confirms the arrival of the order and the consequences for Glory. He threatens Ben on behalf of Glory, but Ben is unruffled and leaves.
Back in her bedroom, Dawn is looking through her diaries. Downstairs Joyce and Buffy are discussing her, and she overhears them. Dawn then trashes her room and tears apart her journals. In the living room, Joyce and Buffy hear an alarm, and Buffy fears that it's the Glory-alert spell in action. Joyce recognizes it as a fire alarm. A fire is burning in the trash can of Dawn's bedroom, which Buffy quickly puts out with a blanket. Joyce realizes and points out that Dawn has run away.
Buffy enlists the help of her friends and Spike to look for the runaway Dawn, who is aimlessly wandering the city.
As Giles and Xander search together, Xander points out all these memories he has about Dawn, including that Dawn has a crush on him.
Buffy is shouting Dawn's name when Spike berates her, pointing out that she'll head the other way if she hears Buffy. Buffy admits that he was right; it is her fault that Dawn took off. Spike reassures Buffy that she'll find Dawn. "And then what?" Buffy asks.
Dawn sees an ambulance traveling down the street, then sneaks into the mental ward of the hospital. Amidst whispers of "it's here," she asks the patients if they know what she is. The Byzantium Knight says, "The Key. I found the Key. Thank you, thank you." He wants to destroy her, but he's tied to the bed. Dawn runs out of the ward and almost plows into Ben the intern. He takes her to the locker room and gives her some cocoa. Ben is concerned. Is Joyce all right? Can he call anyone for Dawn? Her sister? Dawn denies having a sister. Ben understands. He has a sister, too, and she can be a real pain sometimes. Dawn says it's not Buffy, it's her; she's not real. Ben realizes that Dawn is the Key and insists she leave. Ben warns, "She's coming. She's here." And then he morphs into Glory.
Glory changes out of Ben's scrubs into a satin red dress found in his locker. Dawn is confused about Ben and Glory, and Glory doesn't expect Dawn to understand the complications of family. Dawn looks to escape, but Glory threatens her. Dawn is still confused, but realizes that Glory has no clue about the conversation with Ben, and thus does not realize that Dawn is the Key. A guard walks in and informs them that this area is for staff only. Glory effortlessly breaks his neck and escorts Dawn out to a more private place for a long, uninterrupted chat.
The Scooby Gang converges in a park, but no luck and therefore no Dawn. Anything could have happened to her. Buffy suggests checking out the hospital.
In a lab, Glory asks Dawn about the Key, and tries bribing the teenager with puppy dogs and ice cream to start singing. Dawn turns the tables and asks for more information about the Key. If Glory could tell more about it, Dawn would know if she's seen it.
Buffy discovers that Dawn was not brought in to the hospital. When she overhears that a guard has been killed, and that his head was twisted almost clean off his body, Buffy deduces that Glory is around.
Dawn keeps asking questions about the Key, and Glory gives answers. The Key has been around "just this side of forever" and is not evil, but Glory doesn't tell what the Key unlocks. Glory gets frustrated as she realizes that Dawn... has no idea where the Key is. She starts to lose it and is about to suck Dawn's brain when Buffy walks in. A fight ensues. Spike pitches in to fight Glory and gets knocked out. Willow and Tara start chanting. Giles tries to shoot a crossbow bolt, but it just bounces off of Glory. Xander hits her over the head with a crowbar. She's unfazed and disarms him. Glory throws the crowbar at Dawn, and Buffy jumps in the way and catches the crowbar in her chest. Glory says, "Is that the best you little crapnats could muster?" Willow and Tara throw sand on Glory. Willow claps her hands and shouts, "Descadae." Glory disappears. Willow collapses.
Glory reappears high over Sunnydale and falls to the ground as she mutters an obscenity.
Giles is concerned about the powerful teleportation spell the witches did, and Willow, who is obviously hurting, says, "Won't be trying that one again soon." Buffy is concerned about Dawn, who thinks that Buffy doesn't care. But Buffy does care; Dawn is her sister. "No I'm not," Dawn says. Buffy shows Dawn the blood the younger girl has in her hand, and the blood from from Buffy's chest wound. It's Summers blood. Buffy mixes the two together and insists that Dawn is her sister. The two hug. Dawn tells Buffy that Ben was here trying to help her. Buffy reassures her sister that they'll thank him the next time they see him. Right now they need to get home because Mom is freaking out. Dawn is concerned that Joyce will be mad about the fire. "I think you have a sort of get out of jail free card on account of big love and trauma."
Synopsis written by Lori Ann Curley.