Such is the fascinating conceit of “Stowaway,” a nerve-shredding space thriller that starts strong before falling prey to blunter dramatic twists, few of which are as thrilling as the original idea that sets everything in motion. The film’s opening act is an impressive one, with Penna only improving upon the skills that made his last feature, the similarly chilling adventure drama “Arctic,” such a riveting watch. Penna, who wrote the film’s screenplay alongside his “Arctic” collaborator Ryan Morrison, has a real knack for marrying the mundane (want to learn how to open up a space station for business? “Stowaway” will take you through each and every process with care) and the disorienting (queasy early shots of the team rocketing into space are nothing compared to Penna’s choice to never let the audience hear mission control, even during tense conversations with the crew). Strong performances by Penna’s talented cast only aid in crafting an immersive, intriguing opening act.

It’s capped off by the arrival of the eponymous stowaway: ground engineer Michael (Anderson), who tumbles out of an essential piece of life support machinery, injuring both Marina and the ship in the process. Penna stretches his tense set-up to strange ends, and while it’s always clear that some sort of twist is in the offing, “Stowaway” subverts expectations both for better and, as it stumbles toward its final act, for worse. © 2021, Stowaway Productions, LLC, Augenschein Filmproduktion GmbH, RISE Filmproduktion GmbH. All rights reserved. The question of how Michael ended up on the ship is never believably interrogated, and brief moments in which the crew wonder if it’s part of some strange scheme are all too quickly abandoned, particularly by a trio of eggheads who have been trained to over-communicate on anything and everything. It’s an idea Penna and Morrison tease a bit, though, and while the majority of the film is told from Zoe’s perspective, a few dips into Michael’s headspace do little to dispel any concerns about his intentions. Is he a terrified man thrown into a bizarre situation, or are his admissions that he’s pleased to be able to help the crew on their mission proof of something more nefarious at play?

Soon enough, Michael’s provenance is all but forgotten, and the foursome work to make a cramped space livable (and workable) for their newly assembled, extremely high-strung crew. Despite being saddled with little more than a tragic backstory to drive him — Michael’s motives on the ship might be hard to decipher, but he’s certainly got some Earth-bound concerns wearing at him — Anderson does strong work embodying a vague character. Marina, someone clearly secure when it comes to rules, procedures, and processes, begins to crumble as it becomes obvious that the ship (and its inhabitants) is in serious danger. Collette believably adds a low-simmering resentment to her performance, while Kim provides a counter-balance as a beleaguered botanist who struggles to see past his own ambitions. © 2021, Stowaway Productions, LLC, Augenschein Filmproduktion GmbH, RISE Filmproduktion GmbH. All rights reserved. But once the film’s ultimate complication is revealed, “Stowaway” is tasked with flipping from tense, character-driven thriller into hard-hitting drama, a switch that diminishes many of the film’s early strengths. Though Penna’s cast does its damndest to ride the tonal vagaries thrown at them, the movie uses overwrought moral questions to address its biggest mysteries. What’s scarier than space? A bunch of people arguing about life and death within its dark confines, apparently. The issues at hand are already thorny enough, but Penna and Morrison’s attempts to make them startlingly unambiguous threatens to undo all the good work put forth in the film’s first half. Some of that is aided by a pulse-pounding final act that ramps up the tension to new levels, bolstered by a wild idea to save the ship that fits neatly beside such recent space freakouts as “Gravity” and the last act of “The Martian.” The raw nerves are very real, and stellar production design and special effects sell the practical elements of the scheme, though much of it seems engineered to patch over thin writing and wacky emotional upheavals. If it’s this last burst of space-set terror that Penna wants to leave his audience with, that’s understandable, but it’s worth remembering the rocky journey that landed us there in the first place.

Grade: C+

“Stowaway” is now streaming on Netflix.  Sign Up: Stay on top of the latest breaking film and TV news! Sign up for our Email Newsletters here.