At first blush, Marissa Mayer’s nearly two-year tenure at Yahoo seems golden: She has motivated a beleaguered workforce. She has spent nearly $1.4 billion to acquire 37 companies including the social software startup Tumblr. She has launched a slew of new products including a weather app that won praise from Apple designer Jony Ive, digital magazines, and mobile versions of Yahoo Screen, the company’s YouTube-like video property. She has hired celebrity journalists like Katie Couric and former New York Times tech writer David Pogue. Earlier this week, she unveiled a rich assortment of high-budget original programs, partnerships, and planned coverage of live events. And she has finally stopped Yahoo from losing both ad dollars and users.

By another critical measure, Mayer’s time at the company has been a success. Yahoo’s stock has more than doubled in value to $35. But the stock’s strength has little to do with Mayer’s turnaround efforts. Yahoo owes its continued existence to a pair of smart Asian investments. Back in 1996, the company launched Yahoo Japan, a joint venture with Softbank; its 35% stake is worth $9 billion. More significantly, Yahoo YHOO owns 24% of the Chinese Internet dynamo Alibaba. It is expected to soon file for an initial public offering that is likely to be the largest tech IPO in history; Yahoo’s share of the company will probably be worth a whopping $40 billion—impressive until you consider that Yahoo’s total market capitalization is $35 billion. In other words, investors seem to be saying that Yahoo’s core business is worth less than nothing.

And there’s the rub. Yahoo can acquire all kinds of cool technology and generate buzz with video programming, but none of that will solve her biggest problem: Yahoo’s advertising business—which generates four-fifths of its sales—is a mess. In the March 19 issue of Fortune, I wrote about Mayer’s efforts to turn its advertising business around. To report the story, I conducted more than a dozen interviews with Yahoo executives and engineers as well as former employees, advertisers, analysts and shareholders. My goal was to discern the likelihood that Mayer will be able to restore growth to Yahoo’s digital advertising business.

The verdict? You can read all about it here in Fortune’s current cover story.