The billionaire Warren Buffett is just two years away from closing the door on a 10-year wager with a hedge fund manager.

The winner gets $1 million to give to charity, and Buffett's incredibly simple investment idea — just buy one index fund — is way, way ahead in the race.

At the end of 2015, Buffett's position in a Vanguard S&P 500 Index Fund VFINX, -0.83% had posted a return of 65.67%, reports Fortune, while the selection of hedge funds was up 21.87%.

Here's the problem for the hedge fund side of the bet: To win now, the hedge funds need to make up the huge gap between their results and the return on the straight index fund.

That might happen one of two ways: First, there might be a dramatic run higher for the clearly different investment choices made by the hedge fund managers.

If the hedge funds make a series of very astute investments, and if those investments suddenly accelerate in value, it's possible for them to win after all. Two years is a long time in the stock market.

Secret portfolios

The other way the hedge funds can win is if the whole stock market completely falls out of bed, much like what happened in 2008, but — and this is crucial — the hedge fund investments must not crash along with the stock market itself.

Anyone who remembers 2008 knows that the chances of both things happening, a giant stock crash that somehow doesn't affect other investments, is pretty slim indeed.

For a while there, nobody trusted anything. Even money market funds were getting calls from investors seeking to cash out. And money market funds are nearly cash in practice!

Let's say the first scenario comes to pass. The hedge funds have some losers in their secret portfolios but enough massive winners to more than triple their record in short order while Buffett's simple index position treads water.

But that's the problem with active investing in a nutshell. It takes some serious conviction to buy a series of unproven investments and hold on to them for 10 years. Maybe the hedge funds traded a lot over the past few years, seeking a foothold that would allow them to catch up. It's anybody's guess.

Whatever they did it isn't working out, and that can take a serious toll on any investor. Never mind the huge fees charged by hedge funds. Just watching your narrowly invested portfolio badly trail the stock market for years is a hard road indeed.

Winners and losers

And the second scenario? It would take a market crash to really turn the tables for the hedge fund, yet they have to also somehow not participate in those declines.

Even if that happens, they won't really "win." More likely, they'll just lose less and thus be able to declare a somewhat hollow victory.

Is "losing less than someone else" a retirement investment plan you can live with? Probably not, I'd wager. It's cold comfort to hear from your financial advisor that you have to keep working another few years, but a bunch of other people have it theoretically worse.

Retirement investing is not about beating the market. As Buffett knows, great investing is about owning the market at a low cost and letting stocks do the heavy lifting.