Clary is having a nightmare. When she wakes up she thinks Jace is sitting on her bed, but it's really Simon. She tells him about her nightmare. At one point Jace comes around and listens to their conversation, then leaves.
Simon blames Jace for all the craziness, but Clary defends him. Simon mentions something about Magnus Bane as the person who may have stole Clary's memories. Information he got from Camille.
Clary runs off to find Jace. Jace is shirtless and working out. Clary barges in and is immediately embarrassed by his shirtlessness. She turns away while he puts his shirt on. He smirks at her embarrassment.
She asks him about Magnus. They talk.
Izzy comes in to tell them that Simon is leaving.
Clary runs off to stop Simon. Clary tries to convince Simon not to leave. Simon tries to convince her to leave with him. He and Jace have a tiff. Clary tries to justify why she wants to stay with her new friends. Simon leaves.
Clary is sitting around a table with Hodge and the others. He is telling them about Magnus. They don't find Magnus. Magnus finds them. Izzy tells them about a downworlder party they could attend to find Magnus.
Hodge gives them a ruby necklace that Magnus once gave to Camille. The ruby is magic and alerts people to the presence of demons. He tells them to use it as bait for Magnus.
In Chernobyl, Valentine discovers Clave members spying on him and kills them.
Luke is at police station. Alaric admonishes him for being sloppy. He tells Alaric that he's trying to find the Mortal Cup before Valentine.
Clary goes into Izzy's bedroom. Izzy is deciding on what to wear to the party. She gives Clary a dress. Clary thinks it's too tight, but she wears it anyway. Jace walks in and likes what he sees.
Izzy leaves the two alone. Jace doesn't want her to go, but no it's fruitless to try to stop her, so he gives her a seraph dagger to use to protect herself.
At the party, Magnus is talking to a demon. The demon doesn't want him to meet with the shadowhunters, but Magnus is adamant that he will. He orders the demons to set up wards around the party building to protect them from Valentine.
Magnus wants the necklace which is why he is willing to meet with the shadowhunters.
Alec and Izzy are picking out weapons. They talk about the mission. Alec is cranky and doesn't like what they are doing. He vents about Clary and Valentine.
Simon is at home. He gets into bed, but Maureen shows up. Then his mother comes in the room and admonishes him for being out all night with Clary. Maureen is hurt and asks if they are dating. Simon says no and Maureen tells him that she wants to date him.
Then she gets all flustered and backs away, knocking over and breaking a picture of Clary and Simon. She pricks her finger on the glass. Simon is entranced by the blood, but Maureen thinks he is entranced by Clary's pictures and runs off.
Simon tastes the blood from the glass.
Clary and the gang stand in line at Hardtail, the downworlders club. They find Magnus. Jace shows him the necklace and tells him he wants Clary's memories in exchange for the necklace.
Magnus tells them that he fed the memories to a memory demon so he wouldn't end up like Dot.
Clary is upset about Dot. He offers to take Clary with him to escape all the madness, but she refuses.
One of Valentine's men shows up. Alec kills him. Magnus is smitten with Alec. Magnus takes off in a portal, but Clary grabs him and pulls off a button before he disappears.
Outside, Jace and Alec have a fight. Clary confronts Alec. Jace tries to track Magnus with the button, but it doesn't work, so he and Alec do a parabatai track. Clary comments about the intimacy of the act.
The parabatai track is successful. They go to the warehouse where Magnus disappeared to. Valentine's men are all around the warehouse. One of them kills a man whose daughter is nearby. Clary defends the girl. Jace kills the assassin.
Clary brings the girl into Magnus' lair. They are combing through the destruction. He commends Clary for saving the girl. He tells her she is like her mother.
He says he'll summon the demon for her, but she has to make the demand for her memory. But first, he performs some magic to transport his lair somewhere else.
Simon is trying to call Clary to apologize, but she doesn't answer. He leaves a message.
Magnus gives the necklace to Izzy as thanks for helping them defeat Valentine's men.
Magnus asks Izzy questions about Alec.
He takes Clary to another room and gives her some chalk and a drawing. She has to draw the pentagram on the floor.
Jace comes in and draws a rune on Clary to protect her. It's so painful, Clary screams.
The pentagram Clary draws is very elaborate. Magnus tells them they all need to take position around the pentagram and hold hands while he summons the demon. They cannot break the bond.
The demon shows up and requires that they each give up a memory of the person they love the most. Alec gives up a memory of Jace, which freaks him out and he breaks the bond. The demon gets hold of Jace and he's caught in the demon's tornado. Alec and Izzy try to save Jace, but they need Clary's help. Magnus is trying to hold the demon off with magic.
She picks up her dagger. Magnus tells her that if she kills the demon, here memories will be lost forever.
She kills the demon to save Jace.
Jace is stunned then wakes up. Back at the Institute, Jace compliments Clary on her demon killing skills.
She tries to call Simon. Simon doesn't pick up. He is walking towards the vampire's lair. Camille is waiting for him.
She grabs her necklace and has a vision of her mother and Valentine. Valentine senses her presence and tells her he wants the Cup.