Should our descendants want to know about the beleaguered group of individuals collectively pegged as Gen Z, they could do worse than unearth “Dear David,” the new film from director John McPhail. As a teacher who is regularly in contact with “Zoomers,” I found Mr. McPhail’s unironic depiction of their social media-addled, gender-generous, attention-deficited, multi-culti ways true to form. Kind of adorable, too. Mr. McPhail’s take on these kids makes one a tad less cynical about the future.

There’s no room for Boomer sarcasm in the go-get-’em world set forth in “Dear David,” and the film is better for it. Mr. McPhail’s picture brims with — well, if joie de vivre isn’t the right turn of phrase, let me compare its bubbly, pop-wise flavor to those Rock Hudson/Doris Day movies of yore. The young cast is appealing, particularly the effervescent Andrea Bang and the British actor Augustus Prew, who locates a nice balance between vulnerability and neurosis as Adam Ellis, a cartoonist who works at Buzzfeed, an online entertainment platform devoted to the flashy, the trashy, and the glib. 

To those benighted souls for whom the internet is not the summit of all creation, Mr. Ellis and Buzzfeed are actual entities. “Dear David” is yet another film “based on a true story” that, in this case, originated on the social media platform formerly known as Twitter. 

It began with a tweet on August 7, 2017, in which Mr. Ellis claimed that “my apartment is currently being haunted by the ghost of a dead child and he’s trying to kill me.” For the next year or so, Mr. Ellis’s followers thrilled to a series of posts recounting bumps in the night, cats on the prowl, and messages from an unknown caller. It’s a scary place, the internet.

How much credence you lend to Mr. Ellis’s tale of a spirit with “a huge misshapen head that was dented on one side” is a personal matter, but it is the stuff of which spooky movies are made. Working with screenwriter Mike Van Waes, who developed the story with Evan Turner, Mr. McPhail has put together a movie that conjoins comedy and horror seasoned with a bit of rom-com and a soupcon of melodrama. Genre mashing would seem to be the director’s speciality: His “Anna and the Apocalypse” (2017), I gather, is all of the above but with  zombies. It’s also a Christmas musical. Go figure.

“Dear David” is, in many ways, an argument against the prevalence of technology. The film begins with a view of an unidentified street at New York City. It’s 1997. A picturesque blur of atmosphere envelops “Fred’s Groceries.” A lone neon light advertises that the place is “now offering INTERNET.” 

A woman counts cash at the till as her husband rattles on about how “people are going to go crazy” about the web. He uses their son David as an example. Switch to a dank basement in which a blond, wan young boy sits hypnotized in front of a computer. In the space of a minute or so, we are transported from Norman Rockwell territory to “Village of the Damned.”

Cut to 2017 and the offices of Buzzfeed, wherein the head of the company, Bryce (Justin Long), condescends to his employees even as he means to instill their enthusiasm. The scenes that play off the camaraderie of the company’s employees and, along with them, Ellis’s travails with his boyfriend Kyle (René Escobar Jr.), are convincing because they’re emotionally grounded. 

So, too, are the early scenes in which we encounter — or think we encounter — the phantom pestering Ellis. Mr. McPhail’s knack for witty dialogue and quick pacing, aided and abetted here by editor David Arthur, is disarming. Somewhere in heaven, Howard Hawks and Preston Sturges are smiling.

The final third of “Dear David” settles for being a horror film and a horror film only. Mr. McPhail handles himself adroitly, but with more of a straight face and, as a result, a considerable diminution of purpose. Say goodbye to “Pillow Talk” and say hello to something more generic. Perhaps the equivalent of Bryce at Buzzfeed Studios — which, along with Lionsgate, produced the movie — strong-armed the director into settling for expert contrivance in the name of marketability. Whatever the case, a chunk of this movie is as bright and flighty as it wants to be. Gen Z should be proud.