Evil is trying to annihilate the universe....in order to do so they need to cancel the life of Charles Wallace.
Charles unbeknown to most isthe one who keeps the balance of the Earth's galaxy.
When Charles & Louise the Larger (the garden snake) see what seem to be dragons (which in reality is a Cherubim) in the garden, Charles runs to get his sister Meg. When Meg & Charles return to the garden all that remains of the dragons are feathers.
When Charles falls deathly ill due to an internal conflict of his mitochondrial farandolae, which are being taken over by the evil Echthroi, it is up to Meg, the Cherubim, Meg's friend Calvin, & the school principal Mr. Jenkins fight the evil & save Charles Wallace as well as life in this Galaxy.