Terminator Genisys is one of several recent blockbusters that play like hostage notes, cries for help from indentured filmmakers.

This goes back to Avengers: Age of Ultron, in which Tony Stark says, "Sooner or later, you create the thing that you hate" - a line that sounds like something writer-director Joss Whedon might have blurted out in the editing booth after a few beers.

Or consider the plot of Jurassic World: An executive under pressure to maximize box office receipts splices a bunch of dinosaurs together in an effort to create something new and marketable. That's basically the story of The Expendables.

One of them, Arnold Schwarzenegger, turns up in Genisys to reprise his iconic role from the James Cameron original, and in the process enacts every Hollywood star's deepest fear - that eventually, you'll have to contend with a younger version yourself.

Arnold, at least, has fun with it. The movie opens with a re-enactment of the 1984 classic's opening sequence, with naked 1984 Arnold appearing on cue, then set upon by the "older" Arnold, who has crawled through a wrinkle in time.

The grayer, age-lined Arnold is the robot from Terminator 2, the "good" terminator, and he looks older because the human tissue surrounding his robot frame has aged naturally - one of the few plausible bits of backstory we get in Genisys.

The rest is a head-scratcher in which characters from at least four different eras use a machine to criss-cross time. The bottom line: Any person, or any cyborg, in any iteration, can be anywhere, for any reason, which certainly makes things easier for the writers, if not the viewers.

In this context, it's impossible to tell what constitutes a spoiler. Suffice it to say the warped past/present/future alters key assumptions about the original characters - warrior from the post-apocalypse Kyle (Jai Courtney) sent to protect Sarah (Emilia Clarke), who would give birth to John (Jason Clarke), the man who would lead the rebellion against the machines, which in turn send robots to disrupt that sequence.

The revisions that replace these story keystones are bewildering, and worse, alter fundamental aspects of the characters almost entirely.

Sarah's emotionally engaging transformation from vulnerable single woman in the big city to heavily armed maternal bad-ass is scrapped, and she's flattened into a caricature of relentless bravado ("I don't need to be saved!"), one that's become its own dreary, post-feminist cliche.

Kyle still has an important chore to perform - to impregnate Sarah - and this seems unlikely, given the atrocious romantic chemistry between Courtney and Clarke.

The movie's biggest gamble, though, comes in the changes it makes to John Connor, who morphs from a heroic (and messianic) icon to a deadly, crushing bore, presiding over $180 million worth of ho-hum special effects (the Golden Gate Bridge destroyed again, for about the 10th time in the last three years).

"Look what John became," says Sarah, speaking on behalf of cherished movie characters everywhere. "What if that happens to all of us?"