Walter Mitty would have been proud of Gilbert Kaplan, who, when he isn't living his musical fantasy, is an ordinary, everyday millionaire businessman. Just picture the 42-year-old Kaplan onstage at Carnegie Hall. He is conducting a full orchestra and chorus in Mahler's heroic, 90-minute-long Second Symphony. As the last chord fades, the applause builds to the roar of a waterfall. "I turn around to face the audience, and they are on their feet," he says ecstatically. "I feel as if I were floating and out of touch. I hear the thunder, the chorus and orchestra pounding their feet, and I know it has gone beyond my wildest dreams."

Wild, indeed, but it's no dream. Kaplan actually made his Carnegie Hall debut two weeks ago, wielding the guest baton for the American Symphony Orchestra. In fact, he had done it once before; last fall Kaplan paid for the privilege, parting with about $100,000 to hire the orchestra and chorus for a one-night stand at Manhattan's Lincoln Center. Now, at Carnegie Hall, the American Symphony invited him to conduct a benefit concert. So how about that, George Plimpton?

Kaplan reluctantly concedes his amateur status as a maestro. It would be hard not to, since his entire repertoire consists of that single Mahler piece (Resurrection), which he first heard 19 years ago at an American Symphony concert with Leopold Stokowski. "By the time they were halfway through the final movement, I found myself sobbing uncontrollably," he remembers. "From that day I've had a love affair with this symphony. The music kept coming back until it literally haunted me."

His second lasting love affair was with Swedish-born Lena Biorck, whom he married in 1970 and who is the mother of his four children. Still, Mahler's Second remains his mistress. "Fortunately, my wife isn't a jealous woman," says Kaplan, laughing.

He is hard pressed to explain his obsession. "There were certainly no Beethovens in my family," he says. His father, a shirt manufacturer, had a nice baritone voice. His mother, a banker, played some piano. While Gilbert dutifully suffered through piano and French horn lessons, it was his older brother, Joe, who seemed the more musically inclined. (Today, under the professional name of Joseph Brooks, he's a successful filmmaker who also composed the Debby Boone song classic, You Light Up My Life.)

Kaplan made his mark on Wall Street. In 1967, on borrowed funds, he started Institutional Investor, now a top financial journal with annual revenues of more than $25 million. That kind of cash flow provided the initial wherewithal for indulging his passion. Over a period of 14 months he attended—and carefully studied—Mahler performances all over the world. Last year he hired Charles Zachary Bornstein, a struggling young conductor, and worked with him nine hours a day for five weeks at Kaplan's vacation home on Long Island. In the process, he committed the entire symphony to memory. Then, on his magazine's 15th anniversary last September, Kaplan threw a party for 2,800 business friends, hired the American Symphony and the Westminster Symphonic Choir, took the podium at Lincoln Center, and led the musicians through Mahler's difficult work.

The critics have generally been kind, and Kaplan is now a recognized scholar, albeit a somewhat specialized one. "I have two choices," says Kaplan. "I could either stop now with the feeling that I've accomplished what I set out to do, or I could go on." Gustav Mahler, after all, wrote eight other symphonies.