Glinda of Oz

Princess Ozma and Dorothy travel to an obscure corner of the Land of Oz, in order to prevent a war between two local powers, the Skeezers and the Flatheads.

The leaders of the two tribes prove obstinate. Unable to prevent the war, Dorothy and Ozma find themselves imprisoned on the Skeezers' glass-covered island, which has been magically submerged to the bottom of its lake.

Their situation worsens when the warlike queen Coo-ee-oh, who is holding them captive and who alone knows how to raise the island back to the surface of the lake, loses her battle and gets transformed into a swan, forgetting all her magic in the process.

Ozma and Dorothy summon Glinda, who, with help from several magicians and magical assistants, must find a way to raise the island and liberate its trapped inhabitants. (Excerpt from Wikipedia)