The country faces a plague outbreak. In a race against time, the officers from Section D have to track down and quarantine everyone who might be infected, while preventing a panic.

A British plane is thought to have been hijacked and taken to an American airstrip. Ros is captured by the Yalta organisation when the team shows up and they learn of an international cover-up.

As an Algerian extremist escapes his home detention, the team fears he may be plotting a terror attack on US businesses in the UK with the help of local Iranians. However, working with the Americans to stop the attack proves a trying task given their recently-deceitful behaviour.

MI5 has to intercept a Russian arms dealer who plans to pass blueprints for a nuclear device to Iran. Adam and Ana decide to bring their relationship to a close.

Harry discovers there's a mole at large in the Grid. Meanwhile, Ros and Adam board a plane undercover in an attempt to stop Iran becoming a nuclear power.

A live TV broadcast announcing the new peace accord between the UK, US and Iran is hijacked by armed gunmen. Can Adam and Ros defuse the situation before someone's hurt?

Ros is in grave danger after Bob Hogan discovers she's a traitor. Meanwhile, Harry's brought before Yalta's chief, whose identity shocks him to the core.

The team finds themselves suspended from the Grid following what happened involving Ros. Harry learns from an old adversary about a bomb he has planted in London. The team realizes that if they respond their lives will be in danger but if they don't it will be the public who suffers. A shocking discovery is made when they discover who is behind the latest threat.

The team steps in when a British soldier is kidnapped by Al-Qaeda. Meanwhile, an old protégé of Harry's returns to the fold after eight years in a Russian prison - but whose side is he really on?

Harry vows revenge by taking down the head of Russia's intelligence service in Britain with the help of Lucas North. Lucas's loyalties, however, are secretly no longer with MI5 and he exploits Harry's trust to help a Russian submarine destroy Britain's communications system, causing financial and social chaos.

With Lucas back within MI5 he reveals that the Russians interrogated him about Operation Sugar Horse, the most secret ongoing spy operation MI5 ever had. If the Russians know about it, then MI5 has a very high level mole. Harry seeks out his mentor, The Spycatcher, to help him find the mole. Ben is undercover with an Al-Qaeda cell; they are planning to bomb London and what looks like a dry run unexpectedly turns live.

The team is stunned to discover that Al-Qaeda's number three wants to negotiate. It's a risk, but Harry and the team have to take this chance for peace.

The British economy is on the brink of collapse and if one more bank goes down the entire financial system will implode. Ros goes undercover in the City to flush out a dangerous international financier who wants to bankrupt the country. Suddenly, it is not just the economy, but Ros's life that is at risk.

A teenage boy accidentally stumbles upon a government conspiracy and is instantly absorbed into the world of MI5, ending up on the run. Lucas and the team battle to protect the boy from those who wish to silence him.

Harry receives a cryptic message from his asset in Russia which could be the key to bringing down the high-level MI5 mole.

Harry is imprisoned and accused by MI5 of being the Russian mole. Lucas disappears to Moscow in a bid to gather intelligence and get to the bottom of Harry's arrest, whilst the rest of the team tries to prove his innocence on the Grid. But with so much evidence against him and a traitor among the team, things aren't looking good for Harry.

With the Russians launching a nuclear strike on London, the only person who can help the team in this desperate hour is the traitor amongst them. With the Russians hunting them down, the team ushers their traitor through London in a desperate bid to reach the codes, and the bomb, in time.