Kimberly and Mason stole glances at one another in their Freshman and Senior year of high school. While never officially meeting, their inner circles intertwined. While this was not the beginning of their love story, they were behind the scenes in a few other chapters.
The invisible string theory is the idea that two people who are meant to be together, have coincidentally been in the same place, at the same time before even meeting, almost as if connected by a string. Whether by fate, or something greater, Kim and Mason were inevitable meant to be together.