Her body was never discovered, but her his husband, Russell Causley was convicted of her murder and is now serving a life prison sentence for it in England. Their daughter Samantha, 16 years old at the time of her mother's vanishing, and now in her forties and a mother herself, got Williams-Thomas involved, in the hope that he would provide some answers or closure to questions that understandably haunt and torment to her to this very day. Causley has maintained his innocence, but Sam is not so sure.
Dealing as it does with the likely murder of a woman, and the protestation of innocence by the man convicted of it, The Investigator has some resemblances to Making a Murderer, but the two are very different. The body was found in Making a Murderer, so there's no doubt at all that a murder was committed. And while Steven Avery, in Making a Murderer, may be brighter than he looks, he's clearly not some criminal mastermind.
Russell Causley is - or at very least, masterful in how to run Mark Williams-Thomas rather than vice versa in this investigation. Williams-Thomas knows to be wary of everything Causley says, but he can't help responding to them, anyway.
In the end ... well, I don't want to give anything more away, except to say that someone who may also have had a role in the murder may still be at large - if indeed there was a murder.