Big Bird wraps up a playdate with Snuffy, when he hears some strange music. He follows it to the courtyard, where he comes across a box. Inside, Zoe and Elmo are pretending to be aliens. They can't see any traces of life through their periscope, so they exit their ship to investigate. They come face to face with Big Bird, who immediately understands and starts playing along, informing them about Sesame Street. (thetvdb.com)