Review - The "prequel" phenomenon was the result of audiences desperately wanting more of an old favourite, but realizing it had come to a satisfying conclusion: If you can't extend the action at the end, why not look at the events leading up to it?
Trouble is, I can't think of a single time where this has worked.
The Hobbit films, the James Franco Wizard of Oz origin story, the notorious Star Wars prequels - all profitable but all ultimately disappointing.
As Harry Potter predecessors, the reaction to the Fantastic Beasts films starring Eddie Redmayne has been more positive, probably because Potter creator J K Rowling is so closely connected with them.
Unlike Harry Potter, they aren't based on a book, they're original films - and also unlike the Potter films, they're actually written by J K Rowling herself.
The good news is Rowling's fertile imagination shows no sign of flagging. The detail of her wizarding world is always dazzling: the spells, the creatures, the extraordinarily complex backstories, the parade of colourful characters.
In fact, many of the elements in the Fantastic Beasts films were originally throwaway bits in the Potter stories - but many of the lead characters of Fantastic Beasts: The Crimes of Grindelwald seem pale shadows of previous favourites.
Our hero, the floppy-haired naturalist Newt Scamander, is like a self-effacing, grownup Harry Potter. His two offsiders Jacob and Tina are essentially Ron and Hermione given American accents but very little to do.
The villain - the titular Grindelwald - was mostly played in the first film by Colin Farrell before transforming into Johnny Depp, for no convincing reason.
Depp returns in the second film with his patented, slightly iffy British accent and a '30s European agenda: Grindelwald has essentially started a movement to declare war on non-magic Muggles.
Once again, Rowling demonstrates her need of a good editor, as the dialogue rambles and the Nazi parallels are laid on with a trowel.
On social media meanwhile, there's been some concern from the fans about how explicitly this film is going to treat a favourite Rowling character, because at last Fantastic Beasts goes where everyone wanted them to go in the first place - Hogwarts School, and the return of Professor Dumbledore.
Unlike most of the cast, actor Jude Law has something to work with playing the young, sexy Albus Dumbledore - though, when I say 'sexy' I don't mean we see him actually do anything. Don't panic…
A few years ago J K Rowling put the cat among the pigeons - in America at any rate - when she blurted out that Dumbledore was in fact gay and that his feelings for villainous Grindelwald were, shall we say, stronger than mere friendship.
At last, this movie actually hints at that plot point on screen, before weaseling out at the end. This is a project aimed largely at an American audience, after all.
The biggest weakness of the Fantastic Beasts films isn't political nervousness or even a lack of imagination, however. Like the Potter stories, there's almost too much story at times - there's just nothing holding it together this time.
Harry Potter was firmly rooted in the old-fashioned English school story - each book another year, but these ones are just one damn thing after another.
What they needed was an old pro scriptwriter to shape Rowling's brilliant but chaotic flights of fancy into a compelling narrative.
The producers - and Rowling - clearly thought the selling-point was an endless parade of cute CGI Fantastic Beasts. In fact, we may come for the magic beasts, but we stay for the characters.
The fact that we see so little of Tina, Jacob, Queenie - even Dumbledore - is frankly the biggest crime of Grindelwald.