The War against the Rull

Is that a Rull sitting next to you? Now man has conquered all of space and seemingly he is at peace with everone in it. That is, everything except the Rull. Who and what are the Rulls? They are a race so alien and paranoid that they must have originated in another galaxy. In their true form they resemble some sort of dark, large worm with a variety of appendages. But you almost never see a Rull as he really is. The Rull, you see, can change its outward appearance at will, so that even your closest friend or most trusted colleague may suddenly turn out to be a spy in disguise.