Warning! SPOILERS for Stranger Things season 4.
Dr. Brenner's (Matthew Modine) care in Stranger Things.
In season 3 of Stranger Things, Eleven loses her powers after battling the Mind Flayer. Much of season 4, volume 1 follows Eleven's struggles navigating high school bullies without her special abilities. After an incident at the roller skate rink, Dr. Owens (Paul Reiser) comes to Eleven with a proposition to restore her "superhero" status, which involves Dr. Brenner (who survived the Demogorgon attack from season 1), a sensory deprivation tank, and Eleven confronting her traumatic memories of Hawkins Lab.
Season 4 kicks off with a flashback to 1979 and the Hawkins lab massacre, a catastrophic event Eleven is seemingly blamed for. In some of the memories Eleven is forced to relive in later episodes, she sees her younger self in the mirrors around the lab. Instead of using Millie Bobby Brown for these scenes, The Young and the Restless and has been in a handful of short films like The Sugar & Bones Club and Safe.
Blair's performance as Eleven serves as a reminder of how young but mature El actually was during the Hawkins lab massacre. Though Eleven is set up as a potential suspect behind the carnage, due to bullying from fellow patients, it's revealed that a lab orderly - or Number One (Jamie Campbell Bower) - is to blame. Back in 1979, El is small in stature but mighty in abilities, especially when pitted against the imposing Number One. Despite appearances, El successfully banishes Number One to the Upside Down, where he transforms into present-day Vecna.
Stranger Things proves that the Demogorgon, the Mindflayer, and Vecna are not the only roadblocks the characters must contend with, as the show must also contend with time. Even though Stranger Things covers 1983 to 1986, the show's production takes a lot of time and effort. The main child actors are aging faster than their on-screen counterparts, and it makes sense that some de-aging technology may be needed for certain flashbacks and memories.