Downey, Witherspoon for 'Days Before'

Robert Downey Jr and Reese Witherspoon are in talks to star in science fiction movie The Days Before, according to Production Weekly.

The story concerns a race of aliens who travel back in time and invade Earth in an attempt to wipe out the human race. Downey will play a man who must stay one step ahead of the time-travelling extraterrestrials to warn the world of their imminent arrival.

Witherspoon's role is unknown at this point, though the actress recently voiced her desire to star in an action movie.

Timur Bekmambetov, the director behind last summer's Wanted, has been approached to helm the Warner Bros film.