A Star Wars spin-off will tell the story of a mission by a group of resistance fighters to steal plans to the infamous Death Star.

The plot of Rogue One was revealed to fans at the Star Wars Celebration convention in California on April 18.

Rogue One will be the first in a series of on-screen adventures exploring stories outside the core Star Wars saga.

The movie will be set before the first part of the original series, Episode IV: A New Hope.

It was released in 1977 and began with the Empire’s pursuit of a ship carrying Princess Leia and blueprints to the lethal space station.

A specially created trailer was also shown at the convention, featuring a TIE fighter soaring over a planet where the Death Star can be seen in the clouds.

Director Gareth Edwards said that because the Jedi Knights were all but wiped out in the preceding film Episode III: Revenge of the Sith, his story would be different from the other Star Wars movies.

Gareth Edwards said it would not focus on people who were able to use the mysterious “force”.

“It comes down to a group of individuals that don’t have magical powers that have to somehow bring hope to the galaxy,” he said.

Gareth Edwards, a lifelong Star Wars fan, also noted that in the previous movies, characters were clearly defined as good or bad. He said he wanted Rogue One to be more complex.

“The thing that interests me about it, is Star Wars was basically very black and white. Our movie is grey.”

Rogue One starts filming in the UK this summer and is due to be released in December 2016, a year after Star Wars: The Force Awakens.