A woman is digging a grave. This is harder than you might think — it takes forever and you have to displace more than a ton of soil, about the weight of an average hippopotamus, to produce a barely serviceable three-foot-deep hole.

But that effort is not the most arresting thing about the opening of “Something in the Water.” The real question is: Who is being buried in this D.I.Y. pit? (Don’t look at me. You’ll have to find out for yourself.)

A further surprise comes when you clock that the author of this deftly paced, elegantly chilly thriller, Catherine Steadman, appeared on our television screens not so long ago. “Downton Abbey” fans might remember her as Mabel Lane Fox, the drolly acerbic heiress who vied with Lady Mary for the attentions of the less-sexy-than-Matthew-Crawley aristocrat Lord Gillingham.

Among a coterie of tiresome single women and their Bechdel Test-failing dialogue, Mabel stood out for her feisty lack of sentimentality. Steadman brings similar qualities of wit, timing and intelligence to this novel, which has already been optioned by 20th Century Fox (Reese Witherspoon is attached to produce).