Jake, that's not a microscope.

Scientists talk of distant planets that could almost, but not quite, sustain life, lacking some small but essential condition. “Life” (the movie, not the cereal) is a bit like that, an effective sci-fi thriller that would be truly outstanding if it had just a little something more — stronger characters, a standout performance, a cleverer plot — something.

Still, as is, it has a lot to offer, a variation on the “Alien” scenario that isn’t just a ripoff of it. The setting is the International Space Station, where an elite crew of scientists led by Russian commander Katerina Golovkina (Olga Dihovichnaya) have found a single-celled organism in a sample of dirt from Mars. It’s the first concrete proof of extra-terrestrial life, and everyone on Earth is jazzed. Schoolchildren vote to name the organism Calvin.

Hugh Derry (Ariyon Bakare), a British biologist, is in charge of studying the creature in a quarantined lab on the ship. Miranda North (Rebecca Ferguson), representing the Centers for Disease Control, is onboard to make sure the thing neither has nor is space AIDS. Pilot Rory Adams (Ryan Reynolds) is a hotshot and a joker who takes bold risks in the early moments of the film to ensure the safe return of the Mars capsule. Dr. David Jordan (Jake Gyllenhaal), the medical officer, incidentally about to set a new record for consecutive days in orbit, is a thoughtful physician who cares more about people than protocols. The engineer, Sho Murakami (Hiroyuki Sanada), has a wife and newborn baby waiting for him back in Japan.

It will not surprise you to learn that Calvin doesn’t remain single-celled for long, or that not all of the crew members will survive to the end of the movie. Calvin grows quickly into a translucent, gelatinous, tentacled thing the size of your hand, and then bigger still, exhibiting signs of intelligence and problem-solving as it does. It feeds on a lab rat (absorbs it, really) with alarming ferocity, and commits worse crimes thereafter as the crew scrambles to contain it while repairing damage to the ship. The situation leads to several superbly creepy moments, near-misses, and disasters, which director Daniel Espinosa (“Safe House”) executes with workmanlike skill, energetically assisted by Jon Ekstrand’s blaring musical score (“BRAAAAAMMP! BRAAAAAAMP!”). Espinosa takes us on long, zero-gravity tracking shots through the space station, providing a fly-on-the-wall experience. We’ve had this meal before, sure, but it hits the spot.

It’s during a lull in the action midway through that the film that its mild deficiencies become apparent. The screenplay, by “Deadpool” and “Zombieland” scribes Rhett Reese and Paul Wernick, isn’t as humorous or inventive as those credits would lead you to expect, with dialogue that’s merely functional, seldom lively. None of the characters distinguish themselves; though all are given personal, humanizing moments, none of those moments pay off in any meaningful way. The plot isn’t predictable, exactly, but it feels like it needs one more twist or turn or complication to really connect — though I’ll say this much for it: it ends well. With just a bit more ingenuity and horror in the 100 minutes leading up to it, it might have been a new classic instead of a respectable but slight popcorn flick.

B (1 hr., 43 min.; R, a lot of profanity, moderate sci-fi violence, scariness.)