(2.5 stars)

“Encounter” opens with a shot of a bright celestial object streaking across the night sky, followed by ickily close-up footage of insects, particularly the mosquito — or, it is suggested, some mosquito-like E.T. — that pierces the skin with its proboscis and deposits something nasty into the bloodstream. To wit: that something is a nonterrestrial parasitic organism whose signature symptom is “behavioral manipulation” of the host.

That, at least, is according to Malik Khan (Riz Ahmed), the troubled former-Marine protagonist of this film from director Michael Pearce (“Beast”), which starts out like an intriguing sci-fi thriller before morphing into a more conventional family drama. It’s a narrative maneuver that is at once a plot twist and a bit of a bait-and-switch, especially if you were hoping for a straightforward alien-invasion story.

Fortunately, Ahmed (an Oscar nominee for last year’s “Sound of Metal” and more recently seen in the niche “Mogul Mowgli”) delivers another one of his reliably watchable performances. Here, that’s a man unraveling — or perhaps already unraveled — in the face of his quest to combat what he perceives as an existential threat to his family: ex-wife Piya (Janina Gavankar), who may already be infected, and their two young sons, Jay (Lucian-River Chauhan) and Bobby (Aditya Geddada). Shortly after that opening bug sequence, we find Malik stealing into the house Piya shares with her new husband (Misha Collins) and sons, and spiriting away Jay and Bobby.

Okay, that’s kidnapping, as Malik’s parole officer (Octavia Spencer) notes.

Wait: parole officer?

Yes, there’s a backstory here, one that gets unspooled only very slowly. Some would say too slowly, which is also true about certain details of the main story, written by Pearce and Joe Barton, which proceeds without answering some nagging questions until the film is halfway over. Exactly how, for example, did Malik determine that Piya was infected, before driving from Oregon to Nevada with the boys, with whom he hopes to reach sanctuary on a military base? We’re shown that most “victims” of the bug can be identified by anomalies in the iris that can be detected by shining a flashlight in the eye. But Malik performed no such diagnostic test on Piya.

Answers will be forthcoming — they’re part of the aforementioned twist — but to hold them in your head unaddressed is frustrating.

Most of “Encounter,” which benefits only a little from our current state of covid paranoia and government mistrust, is a basic road movie, consisting of Malik’s attempts to reconnect with Jay and Bobby as they evade infected citizens and an FBI agent (Rory Cochrane) in hot pursuit. But it’s also a movie about the boys’ discovery that their father may not be the savior he thinks he is. Nothing original about that. We’ve seen it many times before. But illuminated by Chauhan’s sensitive rendering of Jay’s growing alarm about his father, this portion of “Encounter” brightens an otherwise gloomily familiar dysfunctional family drama.

R. At area theaters; available Dec. 10 on Amazon. Contains crude language and some violence. 108 minutes.