Simon Orwell is a bartender who used to be an electrical engineer. When we first encounter him in Davis Bunn’s novel Unlimited, he’s in a Mexican ditch, fleeing an assassin, trying to protect a device he helped invent.

Simon had a promising career before he self-destructed and betrayed his closest friend, a professor from Mexico. Professor Vasquez’s great dream was to find a way to salvage the energy wasted in electric generation and transmission, to provide cheap energy for the poor. Now he’s been e-mailing Simon, telling him he’s made a breakthrough and wants him to come and join him with his version of the device.

Simon was eager to come. Not so much for the project, though that interests him, but for a more personal reason. To ask Vasquez’s forgiveness.

When he finally reaches his goal, the village of Ojinaga just south of a desolate stretch of the border, he finds that Prof. Vasquez has been murdered. Injured and still pursued by the assassin, he finds refuge in a Christian orphanage, where the director, Harold Finch, a former NASA scientist, has drained his personal fortune caring for the children. He’d like to see Prof. Vasquez’s dream realized as a way to subsidize the orphans’ care. But as much as that, he’d like to see Simon find the answer to his personal torment.

Simon is also befriended by two of Harold’s co-workers, Pedro and Sofia, both of whom grew up in the orphanage. Pedro is assistant to the town mayor, a rising politician who is dating Sofia. Sofia feels obligated to marry the man for the sake of the good he can do the orphanage. But she doesn’t love him, and gradually – against her will – she finds herself drawn to Simon, whom she doesn’t even trust.

Meanwhile drug cartels are showing an interest in Prof. Vasquez’s device. Simon finds himself in a cross-fire, terrified that he’s bringing danger on the first people to care about him in a long time. And he’s not even sure he wants to go on living, bearing the weight of guilt and self-loathing. His new friends offer an answer, but could there really be a power so unlimited?

Unlimited genuinely moved me. The story, if you accept its science fiction premise (which is not that outlandish), is believable, and the characters live and breathe. I wouldn’t rank Bunn in the top tier as a stylist. It seemed to me that time and again he chooses almost the right word, rather than the right word that would really have sung. But your taste may differ. I also suspect that Mexican government corruption, though certainly addressed in the story, is underestimated.

Recommended. Unlimited will go on sale September 1. (A movie of the same title, starring Fred Thompson, is scheduled for release in October.)

Full disclosure: I received a free advance review copy of Unlimited.

Lars Walker is the author of several fantasy novels, the latest of which is an e-book, Hailstone Mountain.