Keanu Reeves stars as a mysterious alien visitor in “The Day the Earth Stood Still” remake.

Keanu Reeves as an alien? Not a stretch. Jennifer Connelly as the empathetic scientist who argues to spare the human race? Again, on-the-nose casting.

“The Day the Earth Stood Still,” a classic parable from 1951 with the most poetic title in all of science fiction, was begging to be updated, and so it has in this new film starring Reeves and Connelly. The new “Day” has spectacular effects to back up its sermon (it was preachy then, it’s preachy now) about humanity’s failings and how those might annoy a universe that’s concerned we’re about to muck up the neighborhood.

But it’s only about three quarters of a good film, a solid thriller that loses heart and ends so abruptly as to make you wonder if the screenwriter didn’t simply go on strike and refuse to provide a finale.

It begins with a 1928 aliens-visit-the-Himalayas prologue and then leaps into a paranoid modern crisis with scientists, including Dr. Helen Benson (Connelly), being “taken into custody” to help the government. What crisis, they aren’t told. Then the object hurtling to Earth slows down and parks in New York and they figure it out. It’s “Close Encounters” time.

An attempt at first contact ends in a shooting; a creature emerges (Reeves) and scares the willies out of everybody, including the flinty secretary of Defense, played by Kathy Bates in humorless “I’m in charge here” mode. Every question earns a creepy, cryptic answer. Where’s he from, what is he and what does he want?

“It would only frighten you.”

He has the same name, Klaatu, and the same gigantic robot, GORT, you almost certainly remember from the original film, updated and made even scarier.

This “Day” first goes astray when Klaatu-Keanu escapes from government custody and meets up with one of his own kind, now in ancient Chinese form (legendary character actor James Hong) and they have a long heart-to-heart about humanity’s virtues and shortcomings — in Chinese, not Reeves’ first language.

But the sympathetic doctor has a stepson (Will Smith’s model-pretty son, Jaden), who acts tough and cracks wise — “School’s canceled, on account of the aliens.”

From time to time, Connelly takes us to the movie this could have been, in a tearful moment when she is confronted by a fellow mother, a soldier, who wants to share her cell phone because they both think the world is ending. Director Scott Derrickson (“The Exorcism of Emily Rose”) could have had a new “Starman” here, if he’d trusted the actors and the emotions of the piece.

In the end, the script and the director got hung up on the sermon, about humanity’s need to change our ways, the hellfire and brimstone punishment that awaits if we don’t. Too bad they settled for a half-hearted “Independence Day.”