Stacking may sound like some sort of Tetris clone, but it gets its name from the Russian stacking dolls that populate its world. You're the smallest of dolls in a set of stacking dolls that make-up a working class family living in a miniature version of an industrialized, early 20th Century city. Dire financial circumstances have forced the family into virtual servitude, and it's up to you to free them from their bondage of labor.
To free your family you'll have to go to their places of work, each of which is a suitable Industrial Age location such as a Grand Central Station style train station, a zeppelin, and a Titanic like steamship. But what can one stacking doll of a little kid do in such magnificent locations? That's where the 'stacking' in Stacking comes in. The stacking dolls in Stacking's world naturally come in different sizes, and you can take control of a doll by getting behind one that's one size bigger than you and jumping inside of it. And once you're in a new doll you're one size bigger, making it possible to stack into the next doll size and so on. You can stack and unstack at will, as long as you always stack one size up and can get behind the doll that you want to stack into. Once stacked, not only can you impersonate the top-most doll, you can use his or her special ability. This is what drives the game's adventure game style puzzle-based gameplay. You need to use the right doll and/or doll ability to get past each obstacle standing between you and your indentured family.
One thing that makes Stacking's puzzles unique is that they each have more than one solution. Once you find a solution to a puzzle it's considered solved and you can move forward through the game. However, you're free to return to it at any time to look for other solutions. In addition to the extra intellectual challenge and game time, your rewards for doing so include earning the game's completion-based achievements and unlocking unique dolls for play. The game lets you know how many solutions there are for each puzzle, and even gives you three levels of hints for each solution. These hints are pretty blatant, and if you don't know how to solve the puzzle after the first hint you'll certainly know how to do so by the third. Most of the puzzles have at least one relatively obvious solution, so if you're just looking to make it to the end of the game you can do so pretty easily even if you don't play many adventure or puzzle games. Some of the solutions can be a little obscure without using the hints, so if you're looking for more of a challenge the game will be able to provide it to you.