Jessica had moved back into her old home at the beginning of Season 4. Her return was mysterious and the Liars felt unsure about why she had returned to Rosewood over a year after her daughter's body was found in her backyard.
Several days before Jessica's death, her identical twin sister Mary Drake came to Rosewood in order in catch a glimpse of her youngest daughter Spencer Hastings, whom she had given up for adoption years before. While breaking into the Hastings house to take a look at Spencer; Mary was spotted by Spencer's father Peter Hastings, whom he at first thought was Jessica, until he realised it was Mary.
Mary begged Peter to help her get revenge on Jessica because she had lied about Mary's eldest child Charles being dead, when he had in fact he had transitioned into Charlotte DiLaurentis. Instead, Peter decided to team up with Jessica to kill Mary, so he could protect his Family.
Mary came to the Hastings house again to talk with Peter, hoping to convince him let her see Spencer. However Peter refused believing her to be unstable and dangerous. Jessica entered the Hastings house at that moment and was shocked to see Mary, she addressed Peter, saying "I thought we were taking care of this". It was then that Mary realised what they were planning and descided to get revenge on both of them.
Day of Death
Jessica was hosting a Bridal Show for charity to raise money for starving children. Detective Holbrook shows up at her house in the morning to update her on the investigation into Alison's murder. He told her that they had a court order to exhume "Alison's" body, as recent evidence suggested that Alison was not actually the one in the grave. Jessica seemed angered by this motion.
While the Bridal Show was taking place, Spencer saw Jessica give a suitcase to somebody in a black hoodie (probably Charlotte DiLaurentis). Jessica was later taken to the police station and informed that Alison was still alive. She shared a conversation with Peter Hastings in which he asked her if they still had an agreement, as the police were asking pointed questions about Spencer. She told him that she could not think about anything else but the fact that her daughter was still alive. Officer Maple then asked to speak to her in private.
Sometime later that evening, Jessica returned home and wrote an email to somebody (presumably Charlotte DiLaurentis) stating "I can't protect you anymore". However, she did not send the email and probably went to speak to whoever it was in person.
At some point, Mary intercepted the medication Peter Hastings was going to supply Jessica to kill Mary and swapped Jessica's low blood pressure pills with Peter's pills, that contained Losartan, which is used for people with high blood pressure. When Jessica took these pills, her heart stopped working. She used Peter's pills in order to frame him for Jessica'a death.
Charlotte, after escaping police custody, snuck home to see Jessica. When she arrived, Jessica's body was lying outside the DiLaurentis house. Charlotte wept over her dead body and begged her to come back. She then dragged her body to the Hastings' back yard and buried her there.
Alison returns to Rosewood the same night and her father embraces her at the police station. He takes her home where Jessica is nowhere to be found. The following morning, a dog, Pepe, is dropped off at the DiLaurentis house, as Jessica had adopted her. Kenneth went to see Ashley Marin to ask her if she could look into Jessica's emails and find out where she was. Hanna later discovered the email Jessica had been meaning to send. Kenneth and Jason argued as Kenneth believed his son knew where Jessica was.Later on, Pepe begins barking at something in the Hastings' backyard. The Liars and Jason go out to find Jessica's hand sticking out of the ground. The police are called and Jessica's body is taken away.
An initial autopsy is unable to determine Jessica's cause of death. Alison later tells Spencer that a toxicology report confirmed somebody switched her blood pressure medication, causing her heart to fail.
- We will find out who killed Jessica in Power Play