Nobita and his friends gathered at Shizuka's house when they heard about the news about missing flocks of bird species. Nobita believed they were taken away by winged creatures and became obsessed on flying on his own. He created mechanical wings and upon the experiment he encountered an actual winged creature flying out of a time-space distortion. Upon helping him whose name is Gusuke, Nobita and his friends were sucked into the distortion and reached Gusuke's home, Birdpia or the world where the winged creatures live. Source: ANN