With its story set "in the near future", meaning at least 60-70 years from now, Ad Astra's protagonist is an astronaut named Roy McBride (Brad Pitt). After a mysterious space anomaly causes a global power surge that kills thousands of people across Earth, Roy is called into a top secret meeting at Space Command – think of it as NASA meets the Army. There he is told that the surge emanated from Neptune and may have something to do with a deep space mission commanded by Roy's father Clifford (Tommy Lee Jones), with whom all contact was lost thirty years earlier. They add that more and even deadlier surges may be on their way if nothing is done.
Although it's not spelled out, one infers from this meeting that McBride père is not only alive but up to no good. Regardless, Roy is ordered to fly to an underground human colony on Mars (via our moon, now well-populated and an active war zone) in order to record an audio plea to transmit to his father.
At this point you might be asking, "Why doesn't Roy just record this message at his home on Earth, then have Space Command beam it to the Mars colony, from where it can be beamed to his father's ship near Neptune? Why does Roy have to go all that way out there, facing deadly perils and famous supporting cast members en route, just to sit in a recording booth for a couple of minutes?" The blunt answer is that if he didn't do this, there'd be no movie. Still, it's a plot hole – one of several – that may make you suspend your disbelief in the proceedings.
If you only look at its thesis that the further away from home you stray, the less human you become, then Ad Astra works. If you want to groove on some cool spacey visuals or indulge your astrophobia, it works there too. (The film is often very creepy.) And frankly, that's enough to make me recommend it. There are lots of interesting ideas at play here. But with its ponderous Malick-like voiceovers, implausibly violent moments, and a thinly-developed dynamic between the father and son characters, Ad Astra's screenplay (by Gray and Ethan Gross) hopes to achieve a balance of epic wonder and intimate psychodrama, but doesn't quite succeed. In a way, it just rehashes the basic premise of old chestnuts like Forbidden Planet, The Black Hole, and so forth: space can make a man crazy.