You have just finished fighting in the wizarding war after defeating Voldemort and destroying all the horcruxes and decided to go travelling with you friends to the muggle world. You go to Hawkins for a holiday and you meet this group of people who are fighting against this other world named the upside down. These people are muggles and you and your friends decided to help them (as you are wizards and witches) go against the creatures in the upside down. They have no idea the wizarding world exists until you help them.