Charlie McGuffin is an optimist, but things are conspiring against him. His brother is in hospital waiting for an operation, his parents are trying to hide how worried they are, and the school bully is upping the ante in Charlie's direction.
The thing is, Charlie's never really been stressed before – not properly, sweatily, heartpumpingly stressed – and with everything going on at home plus all the normal worries at school, he's starting to panic. And this is bad, because Charlie's just learnt that when he gets properly, sweatily, heartpumpingly stressed, he turns in to an animal, all sorts of animals. A flea. A pigeon. A rhino. Who knows what's next?
The school play is only a couple of weeks away, and Charlie is starting to worry. What if he transforms in front of the whole school, while he's on stage playing Sad Potato Number 1? Will everyone think he's a freak? Will he get sent away for Science to deal with? Will his parents crack up with all the extra stress?
With the help of his three best friends, Charlie needs to find a way to deal with his extraordinary new talent. And fast.