The movie stars Emily Blunt as Dr. Lily Houghton, a very Indiana-Jones-like medical doctor in pursuit of a healing tree in the Amazon. For comedic effect, she's brought along her brother MacGregor (Jack Whitehall), and once in the Amazon they hire Frank Wolff (Dwayne Johnson) and his boat to take them up-river. Things become less fact-based as things progress, and the movie eventually earns itself a rating of "Fantasy."
The action is very Disney: improbably perfectly timed rope swings, things like that. It's quite reminiscent of "Pirates of the Caribbean" ... Oh wait, both of them are based on Disney theme park rides.
We eventually find out that MacGregor's impressive dedication to his sister stems from her support of his homosexuality, a story he tells to Frank - who is also totally fine with it. If the movie didn't have other fantasy elements, I think I would have rated it as "Fantasy" for this improbable occurrence (remember, it's set in 1916). Macgregor says "I would have been expelled from society if it hadn't been for her support." I call bullshit: at that time, her support would have had both of them shunned. And then Frank also accepting Macgregor's coming out? Not a chance. But it's very sweet. And of course Disney is simply placing modern mores into their historical-fantasy-adventure movie, something they've always done (although supporting homosexuality has been a long time coming with Disney).
As silly and improbable as the movie is, the action is often fun and many of the jokes are quite good. Johnson and Blunt are a surprisingly good comedic pairing. Whitehall is a decent comedic sidekick: he initially seems destined to be a classical pampered Victorian wimp, but is saved from the worst of the stereotypes by turning out to have a wicked right hook and a semi-sensible attitude to the craziness going on around him.