Quietly Battling for Britain, Without the Tux or the Martini

LONDON— THE nasty twist in the second episode of the new British spy thriller ''MI-5'' comes so suddenly, so shockingly that for a moment viewers may reasonably doubt their own eyes. Up to that point, the story line has been suspenseful and stylish but fairly standard, with the spies from the counterterrorism division of Britain's national security agency working undercover to thwart a plot to foment a race war in Britain.

Then one character, played by a beloved British television star, meets with the most unexpected and appalling fate. The fact that the character's colleagues are required to return unflinchingly to their jobs -- protecting Britain from ruthless terrorists, murderous zealots and other security threats -- illustrates the central drawback of their secretive profession: there is little time for messy emotions like regret or sentimentality.

''MI-5'' has its United States premiere on A&E on Tuesday at 9 p.m. before settling into its regular 10 p.m. slot. In that first episode, ''we wanted to set up the world and lull the audience in by saying here's a thriller that's edgy and urban and interesting,'' David Wolstencroft, the series' creator and main writer, said in a telephone interview from Los Angeles, where he lives and works.

''In the second, we wanted to make sure they knew that this isn't regular film spydom, with Bond jumping off a cliff and into an airplane,'' he continued. ''You're dealing with the scum of the earth here. When our characters put themselves in jeopardy, they really put themselves in jeopardy.''

Still, what made ''MI-5'' such a success when it came out in Britain was its intoxicating blend of the personal and professional. Characters fall in and out of love and lust, befriend and betray the people they investigate, and make agonizing choices between their work and their private lives.

Mr. Wolstencroft said he was inspired by American shows like ''The Sopranos,'' which strive to humanize characters that could easily be portrayed as cardboard caricatures.

''I thought that British television should be doing shows with that kind of ambition,'' Mr. Wolstencroft said. ''I wanted to show the emotional life of spies, to have a contemporary espionage story that had an emotional and human dimension and that at its core was about secrecy. The characters face moral quandaries and quagmires and have to take extremely difficult decisions.''

They have to deceive and dissemble, he added, but ''every now and then they are slightly heroic.''

In one episode, the counterterrorism team -- mostly young, mostly good-looking, led by a charismatically grizzled old-timer -- take on a radical anti-abortion campaigner from the United States who is blowing up targets across Britain. In another, one of the spies, Zoe Reynolds (Keeley Hawes), goes undercover at the Turkish Embassy in London, only to become a hostage when a Kurdish terrorist group seizes control of the embassy. The agents battle Irish terrorists, radical Muslim clerics, would-be suicide bombers, shady international arms dealers and computer geniuses intent on derailing the government, if not the world.

Though the subjects reflect today's headlines, the characters' lives are undoubtedly more interesting than those of the real M.I.5 agents. In ''MI-5'' (the series uses a hyphen to avoid being misread as M-15), the people behind the desks, trawling through the computers and painstakingly putting together the intelligence dossiers, are also the people sent out onto the streets to lead the operations and confront the criminals. And somehow they find time to pursue emotional lives, even though those are doomed to fail. The most spectacular example of this is told through the story of the canny agent Tom Quinn (played by Matthew Macfadyen), who falls in love early in the series with a nonspy named Ellie Simm (Esther Hall), a single mother with a winsome young daughter. What happens to Ellie and Maisie, in an excruciating sequence, can be seen in Episodes 6 and 7.

Even Mr. Macfadyen was not sure, until after the fact, what was to become of that particular story line, which came at the end of the show's first season in Britain. ''All I knew was that I wasn't going to be blown up, because I'd signed on for another series,'' he said in a telephone interview from his house in London.

Tom is idealistic and ruthless, softhearted but professional, fiercely intelligent and racked with guilt over his dual loyalties. Like the other agents, he lives beneath layers of disguises and lies, a trait shared, to a much milder extent, by the actor who plays him.

''As an actor, you present yourself as something you're not as well, and it's kind of attractive,'' Mr. Macfadyen said. ''There is a fascination about disappearing for a while and leaving yourself behind.''

''MI-5,'' a runaway hit in Britain (where it is called ''Spooks''), has made a star of Mr. Macfadyen, whose previous roles were usually in less mainstream projects. Mr. Macfadyen said he had always been interested in the world of spying, mostly through the novels of John le Carré, but that he was attracted to ''MI-5'' because of the intelligence of the scripts. ''They tread the line between Bond and le Carré quite well,'' he said.

Playing the part of Tom has also given Mr. Macfadyen the chance to meet real-life retired spies, from the C.I.A. as well as M.I.5.

''One of the questions we asked them was, 'Why do it?' because the money's rotten and there's no reflected glory,'' he said. ''It seems to be a combination of the challenge and the inverted glamour of it.''

The series has been accused by some critics here of taking itself too seriously, of committing the cardinal British sin of being unable to examine itself with appropriate detachment. But Mr. Macfadyen mentioned a radio interview with the acclaimed playwright Howard Brenton, one of the scriptwriters for the series. Mr. Brenton had pointed out that after each episode, the action just comes to a stop -- that's it, no credits follow -- and then said, ''How self-deprecating is that?''

MI-5

Tuesday at 9 and 10 p.m. on A&E

On the cover (from left): David Oyelowo, Keeley Hawes and Matthew Macfadyen in ''MI-5.''

Photos: From top left: Keeley Hawes as Zoe Reynolds, Jenny Agutter as Tessa Phillips and Peter Firth as Harry Pearce. Bottom left: an ''MI-5'' cameraman with David Oyelowo as Danny Hunter. Right: Matthew Macfadyen as Tom Quinn, with Esther Hall as Ellie Simm (center) and Heather Cave as Maisie Simm. Below right: Megan Dodds as Christine Dale. Bottom: Mr. MacFadyen with Mr. Oyelowo and Natasha Little as Dr. Vicky Westbrook. (Photographs from BBC/A&E)