In times of peace, it is possible to imagine that our private lives are really private, depending on nothing but our own plans and desires. In times of open war, paradoxically, a certain privacy may also become possible, if only in moments of hectic escapism. It is the in-between times, when violence is a universal rumor but not a reality, that make self-enclosed, self-sufficient thought and feeling almost impossible. The 1930s were one of those times, when world affairs, in W.H. Auden’s words, were forever “obsessing our private lives.”

Today, it is becoming clear, we are living through another such period. Since September 11, the ordinary furniture of urban life – the descending airplane, the packed subway car – has become the parable and prey of our obsessions. Every New Yorker, certainly, has had a vision like the one that torments the hero of Ian McEwan’s new novel, “Saturday”: “London, his small part of it, lies wide open, impossible to defend, waiting for its bomb, like a hundred other cities. Rush hour will be a convenient time. It might resemble the Paddington crash – twisted rails, buckled, upraised commuter coaches, stretches handed out through broken windows … Berlin, Paris, Lisbon. The authorities agree, an attack’s inevitable.”

The shift in the public mood since September 11 is so palpable that it practically demands a novelist’s attention. Mr. McEwan – whose last novel, “Atonement,” was a meticulous reimagining of World War II-era England-has decided in “Saturday” (Nan A. Talese/Doubleday, 292 pages, $26) to take the temperature of the age in the most direct manner possible. Adapting the technique used by Virginia Woolf in “Mrs. Dalloway” and Saul Bellow in “Seize the Day,” Mr. McEwan follows the thoughts and actions of one character over one day. But while Woolf and Bellow were mainly interested in their characters’ inner weather, Mr. McEwan uses his fictional surrogate to probe the mood of the city and the times. Perhaps the closest recent parallel to “Saturday” is Don DeLillo’s “Cosmopolis,” but here again the contrasts are telling. DeLillo’s short novel was set in 2000, and diagnosed the plugged-in mania of the Internet boom; Mr. McEwan’s is set in February 2003, and records the depression and uncertainty leading up to the invasion of Iraq.

Mr. McEwan’s sensitive instrument is Henry Perowne, a neurosurgeon in his late 40s. Perowne is deeply loyal to his wife Rosalind, a lawyer, and to his children – Daisy, a poet just about to publish her first book, and Theo, a budding blues musician. Their artistic inclinations are an inheritance from Rosalind’s father, an eminent and egotistical poet named John Grammaticus. On this particular Saturday, the members of Perowne’s family are planning to get together for the first time since Daisy and her grandfather had a bitter quarrel, three years before.

Like Clarissa Dalloway, then, Henry Perowne spends part of his day preparing for a party – buying fish, making a stew, setting champagne to chill. We also see him attending to the other duties and pleasures of a busy, responsible life, which gives Mr. McEwan the opportunity for a series of set pieces: Perowne plays a game of squash with a fellow surgeon, visits his senile mother in a nursing home, watches his son perform a new song. But all these episodes are colored by the atmosphere of political anxiety, which is made concrete, on this Saturday, by the massive anti-war rally that paralyzes the city. Indeed, Perowne’s day begins with an ambiguous portent: Waking up before dawn, he happens to go to the window and sees a plane on fire, streaking towards Heathrow. Throughout the day, he keeps checking the news to learn if the crash is a mere accident, or what Theo sarcastically calls “an attack on our whole way of life” – the new act of terror everyone is secretly dreading.

Unlike the demonstrators, however, and unlike his own children, Perowne is not instinctively opposed to the war. One of his patients is an Iraqi exile who had been tortured by the secret police, giving Perowne a firsthand understanding of Saddam’s regime: “He concluded that viciousness had rarely been more inventive or systematic or widespread.” It is his horror at the dictatorship, more than his fear of Iraqi nuclear weapons, that leads Perowne to tentatively support the invasion. He infuriates his daughter by forcing her to acknowledge that “the price of removing Saddam is war, the price of no war is leaving him in place.” Even more, Daisy objects to his calm acknowledgment of his ambivalence: “It’s true,” Perowne admits, “I honestly think I could be wrong.”

In his moral realism and refusal of easy certainties, Perowne emerges as a genuinely admirable figure, an embodiment of the beleaguered liberal virtues. His exploits as a neurosurgeon, recounted by Mr. McEwan in slightly ostentatious detail, only add to his glamour. As we see Perowne save lives and give sight to the blind, we recognize the amazing achievements and fragility of the secular, scientific mind, now under threat from religious fanaticism. As Darwin said, in a phrase that comes back to Perowne throughout the novel, “there is grandeur in this way of life.”

The plot of “Saturday” is driven forward, however, by a more concrete threat to Perowne’s comfortable “way of life.” Early in the day, he gets into a fender-bender with a thug named Baxter and his two friends. He only manages to escape a serious beating by figuring out that Baxter’s volatility is a symptom of Huntington’s disease, and using the snap diagnosis to overawe his attacker. But as the day goes on, Perowne keeps catching glimpses of Baxter’s red BMW; it is only a matter of time, the reader suspects, before Baxter comes back to take revenge. When it finally arrives, that confrontation – the latest of many coolly horrifying episodes in Mr. McEwan’s fiction – shows that Perowne’s musings on civilization and barbarism are anything but theoretical. How do the sane defend themselves against the insane, the privileged against the hopeless, without succumbing either to cruel arrogance or to timid self-doubt? There are no more pressing questions for us today; by asking them so vividly and scrupulously, Mr. McEwan has written one of the most thoughtful novels of our panicky time.