Prequel to Let Me In and Let Me In 2. Abby and Constance enter Atlanta in 1887. Constance catches a vision of a better life, leading Abby to take up an impossible quest. "Two known nothings can produce an undefined something. That's what we will be. Abby divided by Constance: indeterminate."
The Holy Grail begins in 1887 and develops some of Abby's backstory, specifically her relationship with a schizophrenic named Constance. The initial meeting between Abby and Constance is shown in Let Me In 2 chapter 14. The Holy Grail picks up three days after the Abby-Constance events recounted in LMI2.
Differences in Abby's personality are intentional. The Holy Grail begins 96 years before Let Me In, and shows part of Abby's transformation into the withdrawn, despairing girl Owen eventually meets.