Jake Epping is burned out and lost. His ex-wife has moved on, his students are always distracted, and his novel went nowhere. Then one of his dearest friends, Al Templeton, shows him the rabbit hole, a secret time portal that leads back to 1960. Al asks Jake to head back to the past and create a better world by stopping the Kennedy assassination. Jake heads down the rabbit hole to begin his mission, but finds that changing the past is far more dangerous than he ever would have dreamed.
The "Rabbit Hole" refers to the idiom, "go down the rabbit hole". It means to enter into a situation or begin a process or journey that is particularly strange, problematic, difficult, complex, or chaotic, especially one that becomes increasingly so as it develops or unfolds. It is an allusion to "Alice's Adventures in Wonderland" by Lewis Carroll.