Sam Abbott is supposed to be watching his toddler sister, Amelia, when she disappears on him. Ranger's search-and-rescue skills click into place and he sniffs out Amelia keeping her safe and endearing him to Sam's family. Although he isn't endeared enough to get a name. They call him "Dog" as if they know he is only going to be with them temporarily. The Oregon trail is treacherous and just the place for Ranger. He helps many others on their adventure whether warning the group of danger or saving lives.
Ranger is the main character with the mentality of a dog that adds humor. Sam is comforted by having Ranger because his dog was too old to make the long 2,000 mile trek journey. Others on the trail lose family to disease. Treacherous animals and river crossings add to the excitement. While this is a historical book, there are plenty adventures to keep the reader flipping the pages.
Even though the dog failed to pass the search-and-rescue test, his skills saved many lives on the Oregon trail. The dog doesn't really have a character arc but readers can apply the message of never giving up in their own lives. Tests don't determine a person's character. In an age of accelerated student testing, this subtle message is worth paying attention to. Other themes include, grief, courage, and importance of pets. Add to that, Kate Messner's clean, tight writing and you have a well-paced plot that will be a good addition to any library.
While this story reminds me of the Magic Tree House series in that it has a character that finds an item that magically transports it back in time, I think the text is higher. At 125 pages it is easy to think it is at a lower reading level, but when I looked up the levels the book is placed in grades 4 or 5. Not that those levels are always correct, but concepts might need to be explained to younger readers. It would make a good read aloud.