Stranger Things season 3 left a lot of questions that must be addressed in season 4, among those if and how Eleven will regain her powers – and a fan theory suggests she will need the help of none other than Dr. Brenner. In only three seasons, Stranger Things has left a lot of mysteries that have only grown as the series progresses, such as the experiments and test subjects from Hawkins Lab and everything related to the Upside Down. There are also characters that were believed to be dead but are actually still alive, somewhere, such as season 1’s (human) villain, Dr. Brenner.

Viewers met Dr. Brenner in season 1 of Stranger Things, where an experiment went wrong and opened a gate to another dimension, referred to by the kids as the “Upside Down”. Because of this, Will Byers was kidnapped by a monster called the “Demogorgon” and taken to the Upside Down. Meanwhile, in this dimension, a test subject from Hawkins Lab – later called “Eleven” – escaped, and Dr. Brenner and his team went after her, but not before getting rid of some people who were on their way. Seasons 2 and 3 had different human and monster villains, but Brenner was confirmed to be alive in season 2, though he hasn’t reappeared yet.

Related: Stranger Things Theory: What Season 4's Monster Villain Could Be

Plot details on Stranger Things season 4 are a mystery, which has made way for a bunch of fan theories on how the series will address some of the biggest questions it has left, among those the whereabouts of Dr. Brenner and how he could come back – and one suggests the reason for his comeback will be helping Eleven get her powers back.

What Happened To Dr. Brenner Is A Stranger Things Mystery

Stranger Things Eleven and Dr Brenner In Lab

Dr. Martin Brenner was a senior research scientist and the director of Hawkins National Laboratory. In the 1950s, he was involved in Project MKUltra, a program designed to develop mind-control techniques through heavy experimentation that included the use of psychedelic drugs, sleep deprivation, and physical and mental abuse. When one of the test subjects, Terry Ives, became pregnant, Brenner abducted the child due to her dormant psychokinetic abilities (a result of the experimentation Terry went through while pregnant). Terry was told her baby died, and so Brenner raised Eleven at the lab.

Through a number of tests and experiments, Brenner trained and developed Eleven’s psychic abilities, which he later used to spy on Russian agents. It was during one of those experiments that a gate to the Upside Down was opened, freeing the Demogorgon and creating chaos in the lab, which Eleven used to escape. He then participated in the cover up of Will’s disappearance by placing a fake corpse in the local quarry and sent agents to capture Eleven at Mike’s house. When trying to convince Eleven to go back with him, he was attacked by a Demogorgon and believed to be dead. However, season 2 revealed he survived.

In the controversial episode 7 of Stranger Things season 2, a former agent for the Department of Energy told Eleven and her sister, Kali, that Brenner is still alive and he knew where he is. Kali later used this information to create an illusion of Brenner to taunt Eleven, and producer Shawn Levy later confirmed that Brenner is still alive, somewhere. How he survived the attack of the Demogorgon, where he has been hiding, and what he has been doing is unknown.

Related: Stranger Things Theory: Season 4 Introduces Eleven's Most Powerful Sister

How Dr. Brenner Could Give Eleven Her Powers Back

Millie Bobby Brown as Eleven in Stranger Things Season 3

Dr. Brenner might be the only person who truly knows Eleven’s powers, what they can do, and how to control them. Since his disappearance, Eleven has been trying to learn about her powers by herself (though she had Kali’s guidance in season 2, but it didn’t last long), and after being injured by the Mind Flyer in season 3, she was unable to use her powers again, and by the end of the season, she was pretty much powerless. Because Brenner is the one who helped her train and develop her powers, it’s possible he knows how she can get them back, though the methods might not be ideal, as it could require more experimentation.

Why Eleven Would Turn To Dr. Brenner For Help

Stranger Things: Duffer Brothers Discuss Barb & Brenner's Fates

A fan theory posted on Reddit suggests Eleven will spend some time at a mental hospital (which the author links to a comment done by Dustin in season 1) due to all the trauma she has gone through pretty much her whole life, with Hopper’s supposed death being the last straw. No matter what the writers have planned for Stranger Things season 4 in terms of monsters and villains, Eleven will have to get her powers back if she wants to help her loved ones, and given that she already knows about MKUltra and what her mom went through, she knows that her only hope to get her powers back might be “papa” himself.

Of course, how Brenner is brought back will depend on where he is. Many fans believe he’s either working with the Russians or is another prisoner, just like Hopper, which would make the mission more complicated – though if they find one, they will learn about the other, so consider it a 2x1 package. If not in Russia, Brenner has clearly covered his tracks very well, so finding him won’t be easy, which would make a very interesting subplot.

What This Would Mean For Stranger Things 4

Stranger Things 4 announcement video

Eleven’s reunion with Dr. Brenner won’t be an easy one for her, mentally and emotionally, more so if he returns before she even knows that Hopper is alive – her evil father figure is back, but the good one is still gone. Based on what has been established about him, it’s unlikely he would help Eleven just because, and he would surely ask for something in return, possibly her leaving the Byers (and Hopper) and going back with him, as he wanted at the end of season 1. On the other hand, bringing Brenner back to help Eleven regain her powers could be a way of redeeming the character, if there can be such a thing for a cruel scientist.

Aside from helping Eleven get her powers back, Brenner’s return could be used to answer some of the many questions there are about the Upside Down (especially if he continued researching during his absence) and possibly the Russians’ arrival and plans as well. Stranger Things season 4 has a lot of ground to cover thanks to all the unresolved things it has left so far, and until Netflix reveals a release date, fans will continue to try to answer some of the series’ biggest mysteries the best way they can.

Next: Stranger Things: Every Character Returning In Season 4