For months, we have been laboring to solve a mystery that seems to have no answer. How do we start getting buyers to once again cross the thresholds of homes for sale – that is, to cross them with buy-sell agreements and mortgage commitments in hand?

One need not be Sherlock Holmes to deduce an answer, or Arthur Conan Doyle to write an outline of the twists and turns that lead to dénouement. In fact, we’ve already seen the end of this book, and its conclusion is in the hands of the U.S. Treasury department.

A few weeks ago, the Treasury Department let slip that it is hatching a plan to provide mortgage money for as low as 4.5 percent for new homebuyers. Existing homeowners would not be able to tap into the money to refinance. It would all go to spur consumers into making the commitment that they are currently loath to make: purchasing a home.

Since last Autumn’s Great Financial Debacle, home sales have taken a precipitous dive. Nationally, they are off by about 15 percent. In Massachusetts, they plunged more in the month of November than they have in any single month in years. And median prices here sunk faster than they have in more than the last two decades.

There’s a gigantic clearance sale going on for houses. But consumers aren’t buying.

The answer, for the good of our economy, is not to slash prices further, but to alleviate carrying costs. Mortgage rates at 4.5 percent haven’t been seen since the 1950s. Even skittish buyers will understand that they will hardly see a better deal than this in their lifetimes.

If this plan works, it can be the savior of the economy. Because when people buy houses, they buy lots of things to go into those homes. They buy dishwashers and carpeting, draperies and recliners, televisions and cookware. If we need to restart the consumer spending juggernaut – and we clearly do – this is the way to do it.

Unfortunately, since it leaked its initial plan, the Treasury Department has been unhelpfully quiet on further details. The government has said that any such plan probably won’t be ready until at least February – which would put it in place just in time for the Spring buying season.

The Federal Reserve’s recent action cutting the Fed Funds Rate has already spurred conventional mortgage lenders to bring rates down to the low 5 percent range. That will help buyers – and is plenty of stimulus for refinance prospects who have equity in their homes. But with the dire warnings of economic free fall hanging in the air, more needs to be done.