Meg and Charles Wallace connect with Mr. Murry, their father, who has been trapped and held captive on Camazotz, a planet under IT's control. Through the strength of their love and Meg's kything, they break through IT's influence and establish a direct connection with their father, creating a circuit of love and goodness.
As the circuit of love expands and encompasses all of Camazotz, IT starts to weaken and recede. The Tesseract, a device used for interstellar travel, becomes distorted and destabilized by the immense force of love, ultimately causing its collapse. This collapse results in the destruction of IT and the downfall of its dark reign.
In essence, the darkness that threatened to consume the universe was defeated by the powerful force of love, symbolized by the circuit of love between Meg, Charles Wallace, and Mr. Murry, which served as a counterforce against IT's malevolence.