Matthew Macfadyen as Tom Quinn

Loyal, dedicated and equipped with the sort of steely, indecipherable expression a poker player would kill for, Tom Quinn is the perfect spook. But is he as strong as he appears? And will his gnawing desire for a "normal" life prove his undoing?

Matthew Macfadyen is yet another star to have come from that fabled factory of talent that is the Royal Academy of Dramatic Art. Like his RADA classmate Ioan Gruffudd, Matthew's gone onto become the thinking woman's heartthrob by taking on complex roles like Tom Quinn and Mr Darcy. But, while the Pride and Prejudice movie gained him millions of female fans, he's not averse to rather less attractive roles – see the recent TV drama Secret Life, in which he played a convicted paedophile. A serious actor, then, but also a bit of a giggler on set. According to co-stars, he'd often have the whole Spooks team breaking out into sudden laughter at the most inappropriate times. Bet the director loved that.

Keeley Hawes as Zoe Reynolds

Linguistics expert Zoe Reynolds is an eager and brilliant agent who is committed to serving and defending her country. Yet her very idealism is her greatest weakness, and it won't be long before she must grapple with the dark moral dilemmas of life in the MI5.

She may have the aristocratic poise and English rose beauty of a Keira Knightley, but Keeley Hawes was never a budding luvvie. Her father and brothers are all London cabbies, and, while she did attend the Sylvia Young drama school as a child, it was only because she happened to live round the corner. She actually owes her showbiz career to being spotted by a fashion scout while shopping in Oxford Street, and she made the leap from modelling to acting in the mid-90s with Karaoke – one of the last things written by the legendary Dennis Potter. Since then she's become a bona fide screen queen in everything from Tipping the Velvet to the recent 80s nostalgia-fest Ashes to Ashes.

David Oyelowo as Danny Hunter

A brave but cocky agent, Danny Hunter can be too reckless for his own good – both on the job and in his personal life. At risk of sliding into debt, he also has feelings for Zoe Reynolds that may threaten their professional partnership on the MI5 frontline.

David Oyelowo only started going to drama classes at school because he wanted to get close to a girl he fancied. This was the almost accidental beginning to an acting career that has got critics all over Britain talking. In 2001, at the age of just 24, he was picked by the Royal Shakespeare Company to play Henry VI - the first black man ever to play an English king in a major Shakespeare production. Which wasn't bad for a bloke just two years out of acting school. He was recently seen alongside Forest Whitaker and James McAvoy in the Last King of Scotland, and he's now set to play blues legend Muddy Waters in an upcoming film. Best start clearing space on the mantelpiece for an Oscar or two, then.

Jenny Agutter as Tessa Phillips

One of the most seasoned operatives at MI5, Tessa Phillips's many years as a spook have made her cynical and aloof - but her counter-terrorism experience make her an invaluable guru for younger agents like Zoe Reynolds.

Discovered by agents while at ballet school, Jenny Agutter has been an iconic British beauty for several decades now. Most of her fans will always think of her in connection with the classic children's story The Railway Children, and no wonder. She appeared first in a 60s TV adaptation, then in the celebrated 1970 movie version, and again – playing a grown-up this time – in a more recent adaptation. Her career also includes star turns in landmark sci-fi film Logan's Run and the great horror-comedy An American Werewolf in London. Although clearly the best thing on her CV is Red Dwarf. It's true, she really did turn up in the show in 1993.

Lisa Faulkner as Helen Flynn

She may be a lowly administrative assistant, but Helen Flynn is determined to rise in the MI5 ranks and prove herself just as capable as Tom, Danny and Zoe. But could her vaulting ambition lead her into trouble?

Believe it or not, Lisa Faulkner was talent-spotted while playing a banjo in a tube station. A career as a model soon led her to acting, and she made her debut in the raunchy, tabloid-baiting 1992 film The Lover. Her star continued to rise with Brookside and Holby City (in which her character's violent murder helped make Lisa a household name). She's even dabbled in comedy, appearing in the Fast Show spin-off sitcom Swiss Toni. All in all, she's certainly come a long way since the day she spent camped outside her idol Boy George's home. (She probably won't thank us for telling you about that particular incident...)

Peter Firth as Harry Pearce

Headhunted by MI5 when he was in the army, Harry Pearce is a veteran spook whose strength and wisdom earn everyone's respect. But underneath that resilient façade he's still wounded by his past - particularly his experiences of Irish terrorism during the Troubles.

You may not know the name, but Peter Firth has certainly been a major character actor since the 1970s, appearing in innumerable films and shows including The Hunt for Red October and Pearl Harbor. He first came to everyone's attention in 1973 when he became the very first actor to play the schoolboy lead in controversial play Equus (a role that's recently been reprised on stage by Daniel "Harry Potter" Radcliffe). Firth went onto star alongside Richard Burton in the movie version, which also saw him get naked with future Spooks co-star Jenny Agutter. As Firth later said, "I feel I know Jenny as a best friend. That's what happens when you spend a long time naked together."