Mum has just got a new job as a firelighter in some unheated volcanoes nearby. It doesn't matter to Dad, who's at work all day anyway. And it doesn't matter to Puffing Billy, who just blows smoke rings and backfires all day. But it does matter to Crossfire - a lot. Who will sing his puff-a-bye at bedtime? When their new nanny, Smokescreen, arrives and doesn't do anything like Mummy used to do, Crossfire is quizzical and upset. Gradually, though, as the rapport between the nanny and Crossfire grows, he realizes that being with Smokescreen instead of Mum is lots of fun in a different way.