Talk about a long-distance relationship: Gardner Elliot, a teenager raised in an outpost on Mars, chats online with a Colorado girl named Tulsa in “The Space Between Us.” She doesn’t know his real story, because his existence is a NASA secret. When the two finally meet, they’ll have to take it on the lam like an interplanetary Bonnie and Clyde.

“The Space Between Us” isn’t based on a novel, but it has major potential for all the teenage angst, romance and soul-spilling usually found in young adult fiction. Starring Asa Butterfield as the guileless Gardner and Britt Robertson as the hardened foster-case Tulsa, the movie often delivers on its promises. It is also, however, one clumsily plotted flick, with a screenplay by Allan Loeb that gambles everything on a twist ending you could spot from a galaxy away.

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The back story takes a while to set up. Visionary CEO Nathaniel Shepherd, played by Gary Oldman as an Elon Musk-Steve Jobs hybrid, celebrates the launch of the first pilgrims to Mars, only to learn that one of them, Sarah, was pregnant. Conveniently for the movie, she dies after giving birth to Gardner, whom we later meet as a bright but lonely 16-year-old. Carla Gugino plays Kendra, the closest thing he has to a mother.

So, who’s his father? To find out, Gardner arranges an official trip to Earth but sneaks off to find Tulsa. If their romance never really steams up, it’s because Butterfield’s Gardner is so childlike. It’s more fun to watch him eat his first burger and soak up his first rainfall than to see him slip into a sleeping bag with the girl of his dreams. As they scramble across the country (there’s always a car nearby, or even a plane Tulsa can fly), Gardner’s heart, which labors under Earth’s stronger gravity, becomes a ticking time clock.

Director Peter Chelsom does a nice job showing us Earth’s beauty through Gardner’s eyes — blue oceans, green fields — but the script gets goofier by the minute, culminating in the aforementioned “reveal” and a dangling epilogue that feels highly unsatisfying. “The Space Between Us” doesn’t exactly fail to launch, but it never truly soars, either.