The creators of “Martian Child” seem so excited to have John Cusack headlining their project that they forgot to attend to the rest of the film.

Mr. Cusack delivers a solid performance as a science-fiction writer named David who adopts a young boy who claims to be from Mars. But as it turns out, there is a limit to how long you can stare at the former Lloyd Dobler as he looks confused and heartfelt.

The film is adapted from David Gerrold’s novelette of the same name, which he wrote about adopting his own son. Mr. Gerrold’s book is a quick, sweet read, but the film version has shed some of the plot elements and buckles under the efforts to create a full-length feature.

In the book, Mr. Gerrold, a single, gay sci-fi writer, looks to adopt a child and soon falls for his future son’s picture. Undeterred by warnings of Dennis’s behavioral problems, the writer learns that the young boy thinks he’s a Martian, and David becomes even more determined to adopt him.

The film version sheds the pesky story element of David’s sexuality, setting Mr. Cusack’s character instead as a widower who is asked to adopt Dennis by an agency that he and his wife had contacted years earlier. At first David shies away from the proposal, but soon he finds himself intrigued by the strange boy who spends his days in a cardboard box.

Dennis and David begin a courtship that occupies the majority of the film, and before long, David comes to believe that the boy’s claim may actually be true. The reclusive writer gradually remembers how to appreciate life while trying to help bypass the defensive protections that his new son has created in order to cope with his shifting and difficult life.

But director Menno Meyjes and screenwriters Seth Bass and Jonathan Tolins seem so intent on establishing Dennis’s lovability that they fail to show why anyone would not want to adopt him. Young Bobby Coleman does a capable job of creating a little boy who could be from another world. Dennis, understandably a shy, introverted child, has a slight problem with theft, but his other quirks appear cinematically charming and reasonably motivated.

The film confuses the point by straddling the line of science fiction, trying to keep the audience guessing as to whether it is young Dennis who is delusional, or the adults who are trying to change him. When Dennis finally begins to understand that being a Martian is making life harder for David and for himself, the child asks sadly, “Is it good to be like everyone else?” Dennis doesn’t think so, but the film’s answer seems to be yes. Amanda Peet conveniently lives nearby and plays the lovely overall-wearing Harlee, perfectly accessible when David is ready to start dating again and to create the perfect nuclear family.

Mr. Cusack also brings to the cast his sister, Joan Cusack, who does another in what has become a series of humorous and loving turns as her brother’s sister. Angelica Huston and Oliver Platt are similarly effective but underused. The problem with “Martian Child” is not with the performances, but with the execution of the story. Ultimately, the film occupies a strange place. Watching the unprepared single man interact with a child places it in the company of Adam Sandler’s 1999 comedy, “Big Daddy.” But Dennis and David’s interactions, though often charming, are not funny enough to fit “Martian Child” into the genre of “children’s film,” and the film’s incessant moralism will become too pedantic for adult audiences. Throughout the film, Dennis warns David that he will soon return to Mars. After their first big fight, Dennis decides to follow through on his promise and heads to the observatory, where he claims he was originally dropped off. The ensuing scene is a melodramatic, windblown climax involving helicopters and dizzying ledges.

In the book, Mr. Gerrold sets the same scene with Dennis and his suitcase at a bus stop. The result is sweet and emotionally fraught without stooping to emotional blackmail.

Hollywood has a problem with adding superfluous drama to scripts and employing predictable storylines to help the bottom line, but it’s especially strange when filmmakers and producers take a perfectly charming story and transform it into a film few people will want to see.

mkeane@nysun.com