'Arry and Bert tease Percy for only having a whistle. Percy becomes cross and practices whistling louder out of spite, not realising he's causing trouble for Thomas and Bertie. Percy later regrets it, but can't resist another whistle and whistles so loud that a giant snowball collides into him. However, the next day, he spots a snowdrift on the line, and hears Thomas approaching. Percy uses his whistle to save him and learns how to use a whistle properly.