How long would you survive on a desert island? Most of us wouldn’t outlast our iPhone batteries. That’s why we love movies where intrepid can-do guys are stranded in harsh conditions and make a go of it with just a pocket knife, a gum wrapper and a frazzled shoelace.

Such a man is Mark Watney (Matt Damon), stranded on Mars after his ship departs, his crewmates thinking him dead. (Not their fault, he’s the first to point out.) , And Mars is even worse than your average desert island, where you can at least breath the air and enjoy a seafood diet.

He’s not wholly without resources. The crew left a base camp equipped with oxygen and food that was supposed to last the five of them a month. But the next ship isn’t headed his way for at least four years.

The Martian could have been called Robinson Crusoe on Mars if that title hadn’t already been taken, back in 1964. That was a pretty good movie, but science has come a long way, so the world is due for an update.

At its best, which is the first half of the movie, The Martian gives us exactly what we’ve come to see: Details of a can-do guy doing. That Watney is a botanist helps when it comes to finding more food. Having a passing knowledge of physics and chemistry also proves useful, as science teachers around the world will be glad to see.

Watney is also an upbeat, optimistic kind of guy, or at least that’s how Damon plays him. You just can’t help but root for him.

You don’t ever stop rooting for him, but the spell of the opening scenes fades away. The novel by Andy Weir from which it was adapted is no one’s idea of great literature, but it’s crammed with details: reading it makes you feel that if you were dropped on Mars with a copy of it, you’d be able to get by. Even better, it makes you feel that any problem can be solved if you just apply some rigorous reasoning to it—if you just, as Watney says, “science the shit out of it.”

But books are books and movies are movies, and they don’t play by the same rules. The novel was adapted by Drew Goddard, who has given us such logically wobbly films as The Cabin in the Woods and World War Z. Any hope you have that The Martian might be a science fiction version of Cast Away disappears about halfway through when it cops an attitude  of, OK, that’s enough MacGyver stuff, let’s get on to the standard plot about how we’re going to rescue this guy.

I don’t mean to be spoiling anything here, but there are only two ways this movie can go: (1) Watney learns to love a solitary life on Mars, or (2) Watney gets rescued. I would love to see the first movie, but I wasn’t expecting it. And once it becomes clear that this is going the second route—which happens far too early in the proceedings—it stops being interested in Watney’s survival efforts and concentrates on lots of actors playing stressed out NASA guys. Which we’ve all seen before. (Though if you have to have this kind of stuff, it’s fun to see it done by a cast that includes Jeff Daniels (essentially reprising The Newsroom’s Will McAvoy), Chiwetel Ejiofor, Donald Glover, Jessica Chastain, Kristen Wiig, and Sean Bean.)

It was directed by Ridley Scott, whose reputation as a thinking man’s science fiction director rests on work that is more than three decades old—Blade Runner, Alien. Once the film moves away from the tight focus on one man’s struggle for survival, it becomes lumpy and erratic, lurching from scene to scene as Scott seems to lose interest in anything but making the spaceships look cool.

I’m probably being too harsh. For a night’s worth of entertainment, it’s pretty decent stuff, especially coming after a summer of brainless comic book movies. But The Martian is its own worst enemy: the good stuff at the beginning makes the run-of-the-mill stuff seem worse than it is.