“The Age of Adaline” opens Friday at Point, Eastgate and Star Cinemas. PG-13, 1:53, three stars out of four.

Nicholas Sparks must be kicking himself for not thinking of the idea behind “The Age of Adaline” first. The scribe of “The Notebook” and “The Longest Ride” loves high-concept gimmicky romances that mingle young and old couples together, and “Adaline,” written by J. Mills Goodloe and Salvardor Paskowitz, has a doozy.

And here’s the kicker; “Adaline” is actually pretty good, especially in its second half.

Blake Lively plays Adaline Bowman, a woman born in 1906. At the age of 29, due to a freak mishap (and I do mean “freak”) involving a car accident, a freezing pond and some lightning, she stops aging at 29. She can get hurt or die, but otherwise her body is immune to the aging process. She’s like the dream of the actresses in Amy Schumer’s “Last F—able Day” skit.

But not aging means that she has to watch as everyone else around her does. Adaline becomes adept at moving around, high-tailing it to another town and another identity whenever somebody starts getting suspicious that she looks too good for her age. Her only lasting contact is with her daughter (who grows up to become Ellen Burstyn), but it’s a relationship she can’t publicly acknowledge.

Look, this is silly, and on some level the movie knows it’s silly. (When the storybook-like narration explained the phenomenon would be explained by a scientific theory that wouldn’t be discovered until 2035, I laughed out loud at the sheer audacity of it.) But it’s surprising how much we buy into it, largely because of Lively’s textured performance. She does seem like a woman out of time, friendly but distant, and a little formal. When she talks tenderly and maternally to the 82-year-old Burstyn.

But the movie doesn’t seem to know what to do with its idea for the first half. Adaline plans to leave San Francisco again, only to fall for a kindly philanthropist (Michiel Huisman). Will she tell him the truth, or flee love once again? It all seems melodramatic and inert, with terrible dialogue like “My dad’s head is in the stars — he’s an astronomer!,” despite director Lee Toland Krieger’s (“Celeste and Jesse Forever”) handsome mingling of past and present on the screen.

About halfway through, I was thinking I might bail on “Adaline” and go get some stuff done. But then Harrison Ford enters the film, as a man with a previous connection to Adaline, and the film just knows what it’s supposed to be, suddenly. Ford’s terrific, lived-in performance, as a man suddenly overwhelmed by emotions he forgot he ever had, elevates and sharpens the film. The writing seems better, the other performances seems better, the film cuts to the heart of things. He saves this movie like it was a stranded hiker.

The presence of Ford’s character provides the emotional foundation it was searching for, and “The Age of Adaline” becomes a genuinely poignant look at lost loves and regret. Your move, Sparks.