Ok. For old time's sake.

So, the rules of copyright are conceptually very simple:

- Authors own a copyright over their work for a limited amount of time, then it is escheated to the public domain. While a work is under copyright, you need to get permission to copy it.

- You can't copyright ideas - that requires owning a patent, which has a far higher bar[0] to clear in order to get. You only get copyright over expressions of ideas - at a minimum some combination of uncopyrightables that itself can be considered to have a "thin copyright".

- Works that are "based on" another work are called derivative works. If a work is under copyright, you also need permission to make derivative works. If you got permission, then the new work gets its own separate copyright owned by the new artist.

Now, you would assume that whatever is in the public domain is public domain, right? Well, only sort of. Because derivative works get a fresh shiny new copyright, that casts a shadow on the public domain. So I can publish the original text of Shakespeare's Romeo and Juliet, but that doesn't mean that I can perform West Side Story just because it's a derivative of Shakespeare. But at the same time Jerome Robbins can't sue me for performing Shakespeare. The exact shape of a derivative's copyright is the amount of creativity added, and no more.

Therefore, I can still make my own twist on Shakespeare. But I have to be careful. If I decide "hey let's make our own 1950s New York gang warfare take on Romeo and Juliet", then I'm getting closer to just ripping off West Side Story. In fact, there's even a term-of-art for the minimum quanta of copyright: "thin copyright", which is applied to creative combinations of uncopyrightable elements.

Sherlock Holmes is a series of detective stories published as serial fiction[1]. Notably, the series was ended by the creator killing off Sherlock[2], and then brought back about a decade later. This is known by Sherlock fans as "The Great Hiatus"; and after Sherlock was brought back the author started writing him with a lot more emotion.

Let's go forward about one life plus 70 years ahead of time. You're a descendant of Arthur Conan Doyle and your gravy train is about to end, because people can just use Sherlock Holmes and not pay you anymore. Except that only part of the franchise is in the public domain. Specifically the part before the Great Hiatus. And afterwards, Sherlock is arguably a different character. So obviously, if someone makes a Sherlock Holmes adaptation where he acts like post-Hiatus Sherlock, then clearly it's infringing the copyrighted stories!

And if it weren't for those meddling kids, the estate of Arthur Conan Doyle would have gotten away with it, too. Actually, I'm kinda toning down the original argument. They thought that they could recopyright all of Sherlock Holmes by just owning one of the stories with him in it, which is not how copyright works. "Only copyrighted Sherlock is allowed to emote" was their second argument, which they abandoned when settling with Netflix.

It just occurred to me: next year, Mickey Mouse is public domain. I REPEAT: THERE IS ONLY ONE YEAR UNTIL THE COPYRIGHT ON STEAMBOAT WILLIE EXPIRES. So we're going to see all sorts of litigative fireworks as Disney tries to hold onto that cartoon mouse for dear life.

[0] Copyright is automatic, patents require a filing fee. Please stop laughing.

[1] Specifically, as part of a larger magazine. Victorian Brits subscribed to magazines to read Sherlock Holmes in the same way that Japanese teenagers subscribe to Shonen Jump today to read One Piece.

[2] Which was just as controversial and shocking as, say, a manga ending its run in a magazine today. Sherlock Holmes is basically the ur-fandom that all other fandoms were cloned from.