In Proof, the young but guarded Catherine grieves over the loss of her father, a famous mathematician who had become a legend at the local university for solving complicated proofs, and for suffering from dementia. Just as Catherine begins to give in to her fear that she, too, might suffer from her father's condition, Catherine's older sister Claire returns home to help “settle” family affairs and Hal, one of the father's old students, starts to poke around the house. What Hal discovers in an old speckle-bound notebook brings to light a buried family secret. It tests the sisters' kinship as well as the romantic feelings growing between Catherine and Hal.