You can still watch season four.
The gang was not in tip top shape. Hopper, Joyce, and Murray were trapped in a soviet prison after being freed. Nancy was trapped in Vecna's nightmare spell as Steve tried to wake her up. The true origin of Vecna was discovered by Eleven, who had created him by blasting him into the Upside Down herself. Things are going to get worse if this sneak peek is to be believed.
There isn't a lot of new footage in this preview, but it doesn't look good. Lucas seems to be losing his mind. Even though she escaped in the mid-season finale, Robin seems to be trapped in the Upside Down. The Soviets have been messing with the Upside Down for a long time, as evidenced by the samples Hopper sees. We don't know if the project gave Eleven her powers back.
Vecna declared, "It is over, Eleven" I've been freed by you. You can't stop this right now. Normally, I would dismiss that as normal supervillain talk before the gang reassembles to kick Vecna's ass, but since season five will be Stranger Things' final season, things will not be ending on a high note here.
We will have answers soon. The final two episodes of season five are going to be on the internet.
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