Chris Evans is Frank Adler, who lives in Florida and makes his living repairing boats. He has raised his math prodigy niece (Mckenna Grace as "Mary") for the entirety of her seven year life after the early death of his sister (also a math prodigy). But when he enters her into public school (later than usual as he's been home-schooling her), her skills come to the attention of her domineering grandmother (Lindsay Duncan), who thinks she knows what's best for Mary.
I thought the movie did a respectable job of bringing up the problems of raising a prodigy without being too heavy-handed about how they should be solved. Okay, maybe it's a little heavy-handed - but it had the courtesy to leave the question open long enough that you're given the time to think about it. The dialogue is intelligent, and the characters very well played. If you're going to have a seven year old in a movie, you couldn't do much better than Mckenna Grace - and Evans, Duncan, and the often present Octavia Spencer as their next door neighbour were all reliably good.
The plot summary can be reduced to "man tries to raise math prodigy niece" and just from that you can guess several of the plot points ... but that said, the acting and the writing made this one a treasure for me.