The story of Titan A.E. is not what you would call very original, except for one twist: instead of saving the earth from aliens, it has already been destroyed.

At the beginning of the 31st century, the human race is on the verge of a revolutionary discovery. In fact, they have already built it and named it Titan. This is enough reason for the Drej, an electric blue alien race, to take out earth in order to prevent further evolution of mankind.
Fortunately, a large group of humans escape safely to live the next fifteen years as refugees in space colonies or working lousy jobs. One of them is Cale Tucker (Matt Damon, The Bourne Identity), who's father helpt build the Titan, and who happens to hold the only map to find said machine.
He gets picked up by captain Jospeh Korso (Bill Pullman, Independence Day) and his crew in order to find the Titan with said map and give the human race a new home. Unfortunately, the Drej have also located Cale, which signals the start for a National Treasure In Space kind of treasure hunt, only with less clues and more outer space, which only gets more complicated when it turns out that the captain and his first mate Preed (Nathan Lane, voice of Lion King's Timon) have made a deal with the Drej to deliver them the Titan.
When this is found out, Cale and the pilot Akima (Drew Barrymore, Charlie's Angels) get left behind on one of the space colonies, from which they naturally escape in order to prevent the captain or the Drej to capture the Titan.

Aside from the 'Earth is already gone' story, the big picture is pretty straight forward. There's a map, a treasure, a bad guy and a double cross. So it's not the story that got the high rating. What did the trick is the visual aspect of the film.

The animation of the characters is incredibly fluent. Admittedly, at times a bit too fluent, but nevertheless it's something I haven't seen a lot in non-Disney animation.
Then there's the Drej, an electric blue alien race that seems to be made of, well, electricity, which looks pretty damned awesome. The fact that they're CGI doesn't interfere with the story or the other characters at all. In fact, their bright blue appearance is a perfect contrast against the overall orangey feel of the rest of the film.
But what really takes the cake are the surroundings. They are majestic, to say the least. The clouds which hide the Drej Mothership, the "Hall of Pillars" through which Cale flies the Valkyrie, the ice crystals surrounding the Titan, it all looks spectacular. Who would've thought space was more than just black and burning balls of gas?

In my eyes, Titan A.E. (Stands for 'After Earth') is one of those underrated animations that have really changed the face of the genre. Maybe it's because of the shoddy story, or maybe it's because the leads were Matt Damon and Drew Barrymore, but I get the feeling that Titan A.E. has kind of been shunned aside as if people were afraid to admit that it was made.
Despite of what others may believe, the film lives up to it's name. It is truly a Titan amongst the animation genre.