Stranger Things is one of the most popular shows on Netflix, with its tone ranging from science fiction and horror to comedy to propel the story thanks to a mixture of characters from different backgrounds coming together to save the world. When Will Byers (Noah Schnapp) goes missing, it's up to his three best friends, Mike Wheeler (Finn Wolfhard), Dustin Henderson (Gaten Matarazzo), and Lucas Sinclair (Caleb McLaughlin), to find and save him. Will had entered a dimension known as the Upside Down. His return seems hopeless until the three remaining friends stumble across a young girl in the woods, only identified by a tattoo that reads "011."
Source：https://www.cbr.com/eleven-stranger-things/
