Mark ran away from home, smuggling his dog tucked into a duffel bag aboard a train to Seattle. He's heading for Mount Rainier, which he intends to climb if it's the last thing he does. He's aware of that reality, just as he's aware of the return of his cancer. His parents are a wreck, worrying about him. His best friend suspects where he's gone, but she's sworn to keep his secret. Mark is getting by on the goodness in the world (though he encounters some not-so-good people as well) and the loyalty of his dog, and--for the first and maybe last times--living his life on his own terms. 
 If I had to make comparisons, I'd say we have one part Fault in Our Stars and one part Small as an Elephant, with a dash of Counting by Sevens. I liked this okay, and I'll put it in my library collection, but it's probably not going to stay with me long-term.