It's Heather and Henry's birthday party. And Billie's got cake on his mind! As Billie sets off for the party, Tramcar No. 5 leaves the station. "Don't take a shortcut across the tram tracks!" said Billie's mother. Billie has promised he won't but George says "It will be fine." Who did Billie listen to? Did Billie get some birthday cake? And what happened to the tram? It's Billie's first adventure but will he have a second?