Avatar: The Last Airbender/Recap/Book 2/10 The Library

"Firebenders lose their bending during a solar eclipse!"

Each of the Gaang gets to pick a location for a mini-vacation. Katara's choice, the Misty Palms Oasis, is somewhat less cool than it was a hundred years ago, but they do meet a traveling historian, Professor Zei (Raphael Sbarge), who recognizes Aang as an Air Nomad. He (the scholar) is trying to find a legendary library out in the desert, one reportedly run by the great knowledge spirit Wan Shi Tong. Because of the extensive flying necessary to find it, and the less-than-solid nature of sand as a medium, Toph is essentially reduced to total blindness; she spends most of the episode in the shade of the building, trading good-natured grumbling with Appa.

Inside the Library, Wan Shi Tong allows them free run of the place, but only if they promise not to use any of his knowledge to hurt others. Sokka, unfortunately, comes across a scroll talking about "the darkest day in Fire Nation history," and becomes convinced that he has found a Weaksauce Weakness he and his friends can exploit. By utilizing the library's planetarium, he discovers the obvious pun behind the scroll: the "darkest day" of Fire Nation history was during a solar eclipse, when Firebenders lose the ability to bend. Of course, by now Wan Shi Tong has caught wind of what's going on, and he begins to submerge his Library beneath the sand for all eternity. Sokka, Katara, Aang and Momo flee, abandoning Professor Zei, who elects to stay and learn all he can.

Outside, Toph is having problems of her own. The local Sandbenders have arrived and are intent on capturing Appa. Blind, powerless, basically unable to do anything productive except fight against the (stone) Library's submerging, Toph is unable to prevent the Sandbenders from roping up the sky bison and dragging him away. At the mid-season cliffhanger, the Gaang are now stuck in the middle of a desert with no supplies or transportation.

Tropes:

Abnormal Limb Rotation Range: Wan Shi Tong
Ancient Keeper: Wan Shi Tong
Early-Bird Cameo: Aang finds a scroll with a picture of a lion turtle on it.
- And, of course, the episode's titular Library showed up in Zhao's Villain Rant at the end of last season.
Fantastic Foxes: Wan Shi Tong's knowledge seekers.
Foreshadowing: The chanting that plays while Aang is looking at the lion turtle scroll is the same chanting that is heard at the appearance of the mysterious island in the series finale.
Heh Heh, You Said "X": Aang and Sokka snicker when Professor Zei admires the library's buttresses.
Hey, It's That Voice!!: The professor is Carth Onasi/Kaidan Alenko
I Choose to Stay: Despite it meaning certain death or at best, no way to return to the mortal world, Zei's love of knowledge is too great and he chooses to stay with the library as it goes under.
Myth Arc: continued in this episode. Sokka discovers records of "the darkest day in Fire Nation history," leading him to discover that a solar eclipse is due at the end of the summer.
Ominous Latin Chanting: While Aang is looking at the lion turtle scroll.
One-Winged Angel: Wan Shi Tong intially looks like a regular owl when they meet him. But when he finds out what the group are doing, his form becomes much more larger and menacing.
Owl Be Damned/The Owl-Knowing One: Wan Shi Tong switches from the latter to the former when he finds out that the Gaang is planning on using the information they gained to invade the Fire Nation.
Redundant Researcher: Professor Zei
Sadistic Choice: Toph has to choose between saving Appa from the sandbenders or stopping the library from sinking with the rest of the Gaang inside.
- Although it's not really a "choice". She's blind in the sand, and her few attempts to attack the sandbenders are unaimed and don't hit anything. If she'd chosen to try to save Appa, she'd have failed, and the Gaang would've been buried with the library.
Unusual Euphemism: "I've been training my arrow off."