Paranormal investigators Ed and Lorraine Warren take on one last terrifying case involving mysterious entities they must confront.
As for the new Conjuring movie storyline, nothing so far is known. With The Devil Made Me Do It ending in 1981, it's expected that The Conjuring: Last Rites' story will take place sometime during the 1980s. In 1986, the Warrens had a case that later went on to loosely inspire the 2009 film The Haunting in Connecticut, so the producers may try to avoid the Warrens' more famous cases to keep the series fresh.
Since The Conjuring: Last Rites is being billed as the final installment in the main series, it likely won't be just any old case and should be expected to be climactic.
As with the entire universe of movies, the fourth could play fast and loose with the truth to pump up the Warrens' last big case as something larger than life. More details are expected to come as the film begins to take shape.