Maisie (or Princess Zelda Imelda-Mae as she likes to be called) is grumpy because "it was almost her birthday and she'd commanded her Chief Lady-in-Waiting to organize a Princess Party". The Chief Lady-in-Waiting (or Maisie's mum as she's otherwise known) thankfully agrees to let Maisie have a Princess Party and soon all the other Grumpy Little Girls, Lulu, Fiz and Ruby are invited.
All the girls wear their best princess dresses and Maisie's dramatic tantrum when Ruby arrives wearing a dress more glamorous and "princessy" than her own is an uncannily realistic reminder of a time when being the centre of attention at your birthday party is the most important thing in the world.
It appears that the Princess Party has turned into a bit of a disaster until a little accident pulls Maisie out of her grumpiness and means the Princess Party turns into a completely different occasion.
