The Federal Bureau of Investigation file on Oleg Lyalin offers new insight into what’s been called “the single biggest action taken against Moscow by any western government” - the 1971 expulsion of dozens of Soviet personnel. According to the narrative established at the time, and repeated even in recent publications, Lyalin’s defection “led to the discovery and deportation of 105 Soviet officials who were accused of spying in Britain” and was prompted by a drunk driving arrest. As his FBI file shows, however, the real story is more complicated than that and has long been one of MI-5’s closely held secrets.