Frank Adler (Chris Evans) is a single man raising his seven-year-old niece Mary (Mckenna Grace). Although she was home schooled and would prefer to keep being taught at home, Frank decides to send Mary to school.
Her teacher, Bonnie (Jenny Slate), quickly realizes Mary is a mathematical genius and tells Frank. She and the principal tell Frank his niece would be better off at a school for gifted youngsters. Frank refuses, saying he promised his sister he would give Mary a normal life.
When Frank's mother Evelyn (Lindsay Duncan) finds out, she disagrees with Frank and decides to petition the court to get custody of Mary. Frank, who loves Mary deeply and wants to do what's best for her, fights to retain his rights to raise his little niece.