Secrets abound as the Shadowhunters and Downworlders attempt to return to normal following Valentine's death; Clary struggles with keeping her secret about Raziel's wish; Lilith sets a plan in motion as Simon spends time in the Seelie Court.
The Warlocks' magic becomes corrupted by a demonic presence; Izzy and Luke attempt to track down more information on the recent series of possessions.
Jace suspects that Jonathan is back and behind the mundane attacks; Simon tries to figure out what the Seelie Queen did to him during his time in the glade; Alec decides to host a Lightwood family dinner at Magnus' house.
Jace becomes concerned about his mental state and turns to Luke for information on his family's past; Clary and Izzy go after a rogue vampire; Simon hunts for a new apartment.
Jace tries to learn who is out to get Simon; Clary turns to Luke to find a way to help Jace; Alec worries about the future of his relationship after learning more about Magnus' romantic past.
Clary teams up with Magnus to investigate a recent demon attack; Simon is stunned when he is visited by someone he thought he would never see again; Izzy worries about dinner with her family and a special guest; Luke reaches out to Maryse.
With the Owl's identity revealed, Alec, Isabelle, and Clary head to Alicante to try and find a way to stop him; Luke and Simon team up to track Lilith's latest possible victim; Maia reflects on her past.
Magnus, Alec and Isabelle attempt to reach Jace inside the Owl; Maia and Jordan join Simon on the hunt for Lilith; Luke tries to find a way to get to Clary.
Clary faces down an old foe in an attempt to find answers about Lilith's plan; Alec and Magnus turn to Lorenzo Rey for help, while Lilith finds a new ally; Luke must choose between his pack and his family, as Simon is forced to deal with his past.
The team scrambles to find a way to put a stop to Lilith's plan; Simon makes a heartbreaking choice with Isabelle's help; Magnus reconnects with a powerful person from his past.
Alec, Isabelle and Jace try to come to terms with the apparent loss of Clary; Magnus mourns the loss of his magic and must rely on others for help; Maia tries to help a devastated Simon; Clary finds herself in a new and dangerous predicament.
Simon tries to track down New York's oldest vampire to get more information on the Mark of Cain; now in Paris, Clary desperately attempts to find an escape from her current situation; Magnus tries to live life to the very fullest.
Jordan and the Praetor try to track down Heidi, while the Shadowhunters attempt to locate Jonathan; Luke looks to Maryse for help; Isabelle gets a possible lead on information on what is really going on in the Clave prisons.
Isabelle asks Alec for help as she continues to track down information on the rumors about misconduct at the Clave's downworlder prison; Magnus succumbs to asking Lorenzo for a favor; Jace takes Clary on a special date.
Several members of the team examine the reasoning behind certain rules after consequences to questionable actions come to light.
Clary and Jace come up with a risky plan to get rid of her twinning rune, which may bring an old enemy back into the fray; Alec worries about Magnus.
Simon and Izzy team up on a secret mission to figure out what the Clave is hiding; Clary becomes conflicted about Jonathan as the Shadowhunters try to get answers.
A demon outbreak in New York City on Halloween becomes a top priority at the Institute, but as the Shadowhunters hunt the demons, Jace becomes more worried about Clary; Alec juggles the outbreak with his deep concern about Magnus' depression.
Jace goes under cover to try and save Clary from Jonathan; Isabelle, Simon and Luke race to find a way to finally destroy Clary's bond with Jonathan; Isabelle works to create a solution before Jace's cover is blown.
Alec, Jace and Isabelle make a plan to finally get Clary back and stop Jonathan, much to Luke's and Simon's chagrin; Magnus struggles to deal with his father's reappearance; Maia must make a tough decision that could affect the rest of her life.
Clary comes up with a plan that will bring Shadowhunters and Downworlders together, as Alec struggles for a way to help Magnus.
Jonathan begins his reign of vengeful terror, and the Shadowhunters try to find a way to stop him; Clary must make a sacrifice that could have long-lasting implications for all; wedding bells are in the air for one special couple.