Princess Ozma and Dorothy

Travel to an obscure corner of the Land of Oz to prevent a war between 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)