The Jenna Thing was the accident that occurred while the Liars were in sixth grade. The event resulted in the blinding of classmate Jenna Cavanaugh and her stepbrother Toby's departure from Rosewood. The girls and their friend "Alison" DiLaurentis were the ones who instigated the incident. However, it was later revealed that Jenna and Courtney planned the whole thing.
One April night in sixth grade, Alison DiLaurentis, Hanna Marin, Aria Montgomery, Spencer Hastings and Emily Fields were at Alison's house for one of their many sleepovers. The girls were in the family's living room, giving each other makeovers and trying on each other's clothes. Ali walked past the bay window in boy-shorts when she noticed Toby Cavanaugh right in front of it, staring at them. She and a few of the other girls, also in various states of undress, panicked and screamed. Toby leered at them once more before running off, leaving the girls feeling angry and violated. When discussing what they should do, Ali comes up with a plan to scare her perverted neighbor. She plans to steal one of his fireworks and launch it near his treehouse.
The others are very hesitant about the prank, especially since it had much potential to go wrong. However, Ali successfully convinces them to go along with it and declares herself the one who will pull off the plan. The girls watch her from the window as she sprints over to the Cavanaugh house. The girls are still very uneasy about what their friend is about to do and wonder if she really is going to light the firework. Unfortunately she does and the prank indeed goes terribly wrong. The firework is launched at the treehouse instead of near it, resulting a fire spreading inside the building. Ali promptly returns in a panic stammering that it wasn't her fault.
Now in full-anxiety mode, the girls hear multiple sirens in the distance and Spencer rushes out to see what happened. Ali tries to stop her but she's determined to see what's happened. Still in the DiLaurentises' yard, Ali and Spencer see Jenna being carried out of the burning treehouse and to the ambulance. Toby rushes over to them in a fit of anger, telling Ali that he saw her and that he was planning to tell the cops. Ali retorts that she saw him doing something terrible and that she would tell everyone about it if he ratted her out. Toby, frightened by her threat of revealing his secret, agrees to keep quiet and takes the blame for everything. Spencer is confused as to what Ali saw and insists on hearing the truth. Ali reveals that Toby was RAPING JENNA in the treehouse. She was so shocked at what she was seeing that she didn't pay attention to the angle of the already-lit firework in her hand.
Ali forces Spencer to keep this information quiet and tells the rest of the girls that everything will be alright. She makes friendship bracelets for all of them afterward and makes them promise to never tell anyone about this incident. This is the major turning point of the Pretty Little Liars' friendship with Ali; things slowly but surely start to deteriorate between all of them. Toby is sent off to a reform school in Maine while Jenna attends a school for the blind in Philadelphia.
In "Unbelievable", Aria comes face to face with Jenna four years later in an art class at Hollis College. She is about to confess her involvement in that horrible night, when Jenna interrupts her and states that she and Ali planned the whole thing. Jenna was so scared and appalled by Toby that she wanted him gone. He would often launch fireworks from his treehouse and his parents warned him that another launch would result in sending him away. Jenna utilized that ultimatum and got Ali to help her; though Ali constantly made fun of Jenna, the two girls bonded over having what Jenna called "sibling problems." While Jenna suffered the loss of her eyesight, she tells Aria that she still doesn't regret a thing and is happy that Toby was gone. She also tells Aria that someone else was present that night. Aria, who always thought she saw someone else lurking in the Cavanaugh's yard that night, begs Jenna to tell her. She refuses out of fear and quickly leaves. The mystery person later turns out to be her old friend Mona Vanderwaal, who was also the first "A."
"Why can't we just tell them the truth? Exactly what you saw Toby do, and that it surprised you so much you set the firework off by accident?" -Spencer while questioning Ali's idea of making up an alibi in case the police questioned them.
"What were you guys whispering about the night of her accident? When I woke up and you were in the bathroom?"- Aria Montgomery to Spencer and Ali one year after the Jenna Thing.
"You lit the firework. You hurt your sister." - Ali to Toby after she reveals what she saw him doing to Jenna
"I'll get you. You just wait." Toby threatening Ali after she blackmails him.
"There was a flash of light, and then..this horrible pain. My eyes...my face...it felt like they just melted away. I think I passed out. Afterward, Ali told me she forced Toby to take the blame. Ali thought fast. I'm glad she did. I didn't want her to get in trouble. And it kind of worked out the way I wanted. Toby left. He was out of my life." - Jenna telling Aria the truth about the accident four years later.
"Please Jenna, you don't understand. Something horrible is happening. You have to tell me who else was there!" Aria pleading with Jenna after figuring out the shadowy figure that night must have been A.