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An extremist mullah may be planning England's first suicide bombing. MI5's mole is discovered and tortured, leaving the team in need of someone who can get deep inside the mosque. A former Algerian spy smuggles himself into the UK and offers to help - but his loyalties are questioned by Tom. As the mole gets closer and closer, he falls out of contact with the team, leading to a dramatic showdown in a schoolyard - with one child wearing a bomb.