Hardy's answer was the only one there could be.
"People are unknowable," he whispered to her, "you can never really know what goes on inside someone else's heart."
This then was the core of Broadchurch's drama: you can think you know people, you can think you know your town, you think you can know your family and friends. But in the end, they're all a complete bloody mystery.
It was this of course that made the series so compelling and the job of solving its crime so damned difficult for viewers.
As the story opened, the little town of Broadchurch had seemed idyllic, a handsome beachside community of ordinary, caring folk going about the business of living. Who wouldn't want to visit?
However as we spent more time there we saw beneath the surface and got to know the people of Broadchurch, and their secrets and their lies, as intimately as they knew each other.
Trouble was, the better we got to know them, the more suspects there seemed to be. It was as if the town was being put under an increasingly powerful microscope, magnifying something new, something bad, each week.
In the first or second episode Hardy had asked Ellie whether she was able to metaphorically stand back from her community and to observe with a clear, objective eye what she saw. She said yes, but in the end she couldn't.
The person who knew Broadchurch best, Ellie, could not solve the crime because she knew the place too well - and, as it turned out, not at all. Instead it was the outsider Hardy who put the pieces together.
And what a relief that those pieces made a satisfying whole. I have to confess my heart sank a little last night when the culprit was revealed with such apparent ease so early. Here we go, I thought, an unworthy ending to a great story.
Thank heavens it was not, although last night wasn't a perfect conclusion. Why the mad woman in the caravan, Susan, took Danny's skateboard and hid in a cupboard was never satisfactorily explained (or not to me). And Vicky McClure's tabloid journalist Karen was a character who seemed to go nowhere.
Yet in total this was a very clever piece of work with excellent direction and some utterly compelling performances (particularly from Olivia Colman as Ellie) built on a terrific script.
The best of Broadchurch was some of the best television I've watched in recent years, not least the scenes last night where David Tennant's Hardy had to reveal first to Danny's family and then to Ellie who had done the crime.
It was smart stuff too - in this time of hysteria about pedophilia - that the nature of Joe's obsession and relationship with Danny was painted in shades of grey rather than in black and white.
In a television age now so dominated by the incredible - but often sprawling - work coming out of America's cable networks, Broadchurch proves there's still a place for short stories well told.
