The story begins with Ginny's family moving east on the adventurous whim of her parents. She's a new student for her senior year of high school, and she feels pretty down about having no friends and her older brother/BFF Paul leaving for college. But Ginny quickly becomes friends with her neighbor and classmate, Caulder, who introduces her to "The Alien" in the class, Smitty Tibbs. Most everyone in town has gotten used to Smitty, but Ginny becomes fascinated by him. He doesn't speak and he doesn't interact with anybody. He's not stupid--he's actually quite the genius--but he just doesn't talk.
So Caulder, who's been looking out for Smitty for years, recruits Ginny to help him pull Smitty out of his shell. First, they have him help Ginny with her math homework by having him write out step-by-step instructions. They then invite Smitty to go with them to the local film society where they screen old classics. All of this is much to the chagrin of Smitty's mother, who seems confused that anyone would try to befriend her clearly disabled son. But eventually, the two of them, although mostly Ginny, are able to break through the barriers that Smitty has kept up for so long.
If this sounds a little made for TV, it's a little like that. But the characters are written so fully, and the relationships are so organically grown, it rises above what could have been a saccharine plot. Ginny as a character just makes sense to me. Her doubts about herself, her fears at adjusting to a new life, and her relationship to her family seem natural. I'm mostly making this point because I read this book shortly before reading the first Twilight book which also features a "new girl becomes intrigued by weird but pretty guy everyone ignores and begins a tenuous relationship with" plot, but with a dislikable protagonist who is only defined by a list of dislikes. Unfortunately, I think I imprinted my love of The Only Alien on the Planet to Twilight which fooled me into thinking it wasn't that bad of the read. But I can't reread more than a couple chapters of Twilight, even though I could reread The Only Alien on the Planet hundreds of times.
Granted, the latter half of Kristen D. Randle's novel may be oversimplified, but it's genuine, never predictable, and you go with it because the characters are so interesting. For me, it's a page turner and a fully inhabitable book.