The fourth series of Spooks began transmission on Monday 12 September 2005 on BBC One at 9 pm with the first of a two part story. The next day (13 September) the second episode was shown. The following week Spooks assumed a 9 pm Thursday slot, a break from the Monday 9 pm slot the previous series had traditionally occupied. Once again the series ran for 10 episodes and averaged 6.05 million viewers, a notable increase on the previous series.

The opening two parter provided the series with two new regulars in Zafar Younis (Raza Jaffrey, whose character had debuted in the final episode of series three) and Juliet Shaw (Anna Chancellor). However it was also a controversial storyline as it featured terrorists (albeit anti-humanity and technology extremists rather than al Qaeda) bombing central London, something that in reality had taken place two months earlier on 7 July after the episodes had already been shot.

According to The Guardian newspaper the day the first episode aired, "The similarities were sufficient to cause head of drama Jane Tranter and new BBC One controller Peter Fincham to agonise over whether to drop the episodes." The episodes eventually aired unedited, although before both installments of the two-parter the BBC One continuity announcer warned viewers that they featured scenes of terrorist bombing in London which some viewers might find disturbing.

Episode seven saw the departure of Fiona Carter, as actress Olga Sosnovska was pregnant during filming and elected to leave the programme. In a by-now traditional shocking exit Carter attempted to kill her deranged ex-husband, who she thought had been hanged several years previously. Her ex-husband, however, abducted her and later shot her dead in Adam's presence when she tried to escape (by cutting her own wrists with broken glass to fake a suicide attempt, and thus managing to overpower her guards temporarily). Her character was replaced in the Spooks set-up by Miranda Raison as Jo Portman, a new arrival at MI5 who had been recruited by Adam in the previous episode.

Episode nine also introduced the idea of extraordinary rendition to British television. The episode, entitled The Sting, once again displayed the programme's ability to deal with delicate subjects and hinted at the levels of subterfuge in international politics.

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