As others have written, Sister Boniface "just is" and, except for a subtle hint about her earlier work with MI5 or whatever, there is no acceptable reason she runs the show for the cops. As well, the story resolutions are in the vein of "then a miracle happens" with something evidentiary.
This show makes the Father Brown shows seem like Grantchester; in short, there's much less here than meets the eye. Pity.
This show makes the Father Brown shows seem like Grantchester; in short, there's much less here than meets the eye. Pity.