1. THE WORLD OF SPOOKS
"Spooks" are spies, and there are rather a lot of them in this show. Unlike most espionage films and TV shows, which like to focus on a central charismatic character (whether that's James Bond or George Smiley), Spooks is a sprawling ensemble set-up, with conflicting personalities working together to keep Britain safe from every nightmarish threat you can think of. Their London headquarters are known as The Grid: a warren of computers and corridors which is the nerve centre of their counter-terrorism efforts. While the spooks are a tight-knit bunch, there are rivalries and conflicts everywhere, and they have to keep their wits about them because traitors and double agents lurk everywhere.
2. NOBODY IS SAFE
One thing that truly sets Spooks apart from almost all other major, long-running dramas is the fact that everybody in this show is expendable. Even the central figures, the big stars we assume will always be impervious to real harm, can meet their grisly doom out of the blue. One notorious episode, relatively early in the run, saw an apparently major character brutally executed before our eyes - a moment so shocking it triggered a flood of complaints to the BBC. But that's Spooks for you. It's a bracingly fearless rollercoaster of a show where the good guys aren't always snatched away from the jaws of disaster, and where nobody is assumed to be safe from the villains. Rather like real life, in other words.
3. THE TEAM CHANGES
Over the course of its epic run, Spooks has shaken up its central team a fair few times, and each "generation" is headed up by a very different leading character. The main man of the first few series is Tom Quinn, played by the doe-eyed Matthew Macfadyen. He's the very picture of the cold, emotionally detached spy, at least on the surface. Beneath it all, he's a hotbed of moral confusion. His successor, Adam Carter (Rupert Penry-Jones) couldn't be more different. He's the quintessential action man and heartthrob, and a former MI6 officer to boot. Meanwhile, the most enigmatic Spooks era is that of Lucas North, played by the raven-haired Richard Armitage, who has served years behind bars in Russia, and may prove as dangerous as the very bad guys he's up against.
4. IT TAKES ON THE BIGGEST SUBJECTS
Spooks could never be accused of playing it safe, story-wise. This is a show which has covered everything from the rise of white supremacists to the threat of a newly resurgent Russia. It also has plots concerning Islamist radicalization, pro-life terrorism, secret aggression from Iran, portable nuclear bombs being smuggled into London, and - in one of the most gripping story arcs - a secret conspiracy by members of the British establishment to create an authoritarian Big Brother state. If you weren't paranoid before, you will be after watching a few episodes of Spooks.
5. PERSONAL LIVES MATTER
Spooks isn't just about apocalyptic terror threats, lethal racist fanatics, high level corruption and evil politicians smirking into their brandy glasses. It's also about the daily grind of spy work, and the toll it takes on those who essentially allow themselves to be owned by MI5. Their personal lives are often as fascinating as the cases the agents work on, whether it's Lucas North's personal traumas or Tom Quinn having to lie about his day job to the woman he loves. The questions raised, about loyalty to your country versus loyalty to your loved ones, will linger in your mind after the credits roll.
6. IT HAS CUMBERBATCH
And to round things off, special mention must be made of the presence of a certain future superstar. Yes, Benedict Cumberbatch turns up in Spooks in one of his earliest screen roles as Jim North, a government employee whose poor choices place him on the radar of MI5. On the subject of famous faces, look out as well for such eminent thespians as Hugh Laurie, Andy Serkis, David Oyelowo and Simon Russell Beale.