The Moonstone is composed of a series of first-person narratives, memos, letters, and journal entries. The bulk of it is told retrospectively, from the points of view of the people involved in the action. Franklin Blake, one of the main characters, is responsible for collecting all the documents, and Gabriel Betteredge, the first narrator, quotes Franklin Blake's explanation on the first page of his narrative:
'There can be no doubt that this strange family story of ours ought to be told. And I think, Betteredge, Mr. Bruff and I together have hit on the right way of telling it. […] We have certain events to relate, […] and we have certain persons concerned in those events who are capable of relating them. Starting from these plain facts, the idea is that we should all write the story of the Moonstone in turn—as far as our own personal experience extends, and no farther. (220.127.116.11-8)
So the first assertion Franklin Blake makes is that the story "ought to be told." He thinks that it's important for future generations of their family to know the truth about the Moonstone. He also thinks that there's a "right way of telling it." Lucky us! We're sitting down to read a story that needs to be told, and we're going to hear it the only "right way" that it could be told. Of course, Franklin Blake wants to make sure that the bare facts are told – the reader (his imagined future generations) should be allowed to judge for him or herself. So that puts us in the position of detective – we hear the events just as the characters themselves experienced them, and we know only what they knew at the time.