The Doctor used a Metebelis crystal to help Stevens regain control of his mind. Stevens then seemingly sacrificed himself by triggering an explosion which destroyed BOSS and the refinery. (TV: The Green Death)
However, it transpired that Stevens and BOSS had survived and their reported deaths had been part of a government coverup. Twenty years later Stevens re-entered public life as Director-General of the BBC.
Stevens remained under the influence of BOSS, who in the years since the Llanfairfach incident had become interested in interactive and digital broadcasting as a means of spreading his influence. (HOMEVID: Global Conspiracy?)