The Lego Movie is the story of Emmett, the most regular of regular Joes. Emmett lives in the city, where he's a construction worker, working to the Instructions in his own unremarkable way. But then one day, completely by chance, he finds the "Piece of Resistance", is hailed as the "Chosen One" and dragged into a resistance movement against the evil President (Lord) Business who intends to freeze all the Lego Worlds into perfect statis. At this point, this is a pretty generic plot, that the film spends a lot of time them trying to subvert in a number of ways.
For a start it quickly becomes apparent that the "Chosen One" prophecy is bunk. Emmett doesn't have any sort of special destiny he's just the wrong guy in the wrong place. He can't make the big speech to bring everyone together, he doesn't have any innate "Master Builder" ability that is any better than any other character who spent time trying to learn it. Emmett normality is the butt of many jokes but its also one of the films main points; that anyone can do this sort thing. It allows The Lego Movie to get away with following a host of familiar beats whilst at the same time poking fun at them, a wry tone that it gets a lot of mileage from.
It's a film that also makes great use of it's core property. Not only are all the sets made out of Lego blocks, but nearly every major set-piece makes (sorry) use of the fact that nearly anything can be torn down and turned into something else. Characters have a matrix-like ability to "see" the Parts around them, and recombine them into new things, all done in a fake "stop-motion" building effect with that little plastic clicking noise that Lego fans will know and love. Shout-outs to Lego properties abound, both small and large, but they never get in the way of the main story and it's new characters (and Batman)./
Without being too spoilery, the film does take a turn I didn't expect in the last act and is all the better for it. It was always going to be hard to get an emotional resonance into a movie about plastic minifigs, and the choices made to get that emotional effect are, I think, brave (because they could have worked horribly) and right. It speaks of a film that understands that whilst it may be there to sell Lego sets, its needs also to sell the idea of Lego, of what it is and why it has become such an institution in so many childrens lives. And that's why it works.
Now if you'll excuse me, I'm off to build another Batmobile.