MARTIAN CHILD — ** — John Cusack, Bobby Coleman, Amanda Peet; rated PG (vulgarity, mild profanity, slurs, brief drugs)

Somehow, "Martian Child" makes us care about the title character during this film's first 15 or 20 minutes, even though we don't get to see very much of for him.

He's Dennis (Bobby Coleman), a 6-year-old orphan who conceals himself under a cardboard box and who claims to be an extraterrestrial life-form.

The film's early sections, in which this quirky character eventually comes out of that self-imposed isolation, are by far the best part of this comedy-drama, which is based on best-selling (fictionalized) memoirs of science-fiction/fantasy writer David Gerrold.

Unfortunately, the second half of the movie gets pretty messy, and the sudden tonal shifts (things go from light comedy to dark drama) are a bit jarring.

The fictional version of Gerrold is novelist David Gordon, played by John Cusack. He's recovering from the loss of his fiance but is looking to open up his heart and home to another person. So he's decided to adopt a child, though his agent (Oliver Platt) and sister (Joan Cusack) both think he might not be ready for that kind of commitment.

They're even more concerned when they finally meet the boy he's decided to adopt — a boy who claims to be a Martian, who doesn't relate well to others and has a kleptomania problem. It's no wonder the case worker (Richard Schiff) is thinking of taking Dennis back to the group home for his own good.

Aaron Zigman's remarkably unsubtle musical score is a dominating presence, especially in the more comic scenes. And screenwriters Seth Bass and Jonathan Tolins never bother to explain the exact nature of the relationship between Cusack's character and one played by Amanda Peet.

That the film evens succeeds to this degree is a tribute to the cast, especially the believable onscreen relationship that develops between Cusack and relative newcomer Coleman.

The supporting cast is terrific, including Cusack's immensely likable real-life sister, Platt and Sophie Okonedo, who plays a group-home supervisor.

"Martian Child" is rated PG for for some mildly suggestive language and references, and mild crudities, mild profanity (mostly religiously based), violent content (a food fight and child-in-peril elements), slurs based on disabilities, and brief drug references. Running time: 108 minutes.

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E-mail: jeff@desnews.com