Underdog is guarding a train delivering gold to Fort Knox. Riff Raff and his gang have captured Polly, tied her up and placed her in a free-flying balloon amidst several other balloons equipped with recordings of her voice. Underdog hears his true love's calls for help and flies to the area only to be confounded by the multiple balloons. At the train, Riff Raff and his men force the station master to reroute the gold train to an express track, and a potentially deadly crash.