In this middle grade/YA sci-fi novel, Zach and Jenna are two ordinary siblings, until a portal opens in their kitchen, swallowing up their parents. When they decide to go in after them, they find themselves transported from one world to another, having to rely on their courage, intelligence, and one another in order to survive each dangerous new situation.At first, I had a hard time getting into this book. It seemed fragmented, non-cohesive, and at times somewhat ridiculous. The siblings seemed to flit around from one world to the next, following a predictable pattern of new world... mortal peril... problem-solving... success. At some point, though, it started to remind me of the classic literary adventure of Milo in The Phantom Tollbooth by Norman Juster, and from that point on, I really started to enjoy their adventure more -- perhaps I just needed to take it a little less seriously?As soon as I started seeing it more as a series of small adventures, it became more enjoyable. In fact, I discovered that most of the things I thought were ridiculous or unrealistically improbable or downright inconsistent were in fact, that way on purpose. I don't want to give too much away, but I thought myself rather clever when the siblings finally figured out what was really going on.For instance, at one point there are creatures who only understand language in the most literal sense, yet one of them describes magic and science being "two sides to the same coin." Later, this inconsistency makes a lot more sense, and could even be considered a clue, if Zach and Jenna had been observant enough to notice it.There is still some repetitiveness to it, and some things that still don't entirely make sense (such as the two kids jumping in the portal in the first place), but overall, it was an interesting journey, full of riddles, mind-bogglers, and logic puzzles -- a great challenge for those who'd like to sharpen their minds!Thanks to the publisher for providing me with a review copy of this book!