Based on the book by Gayle Forman, R.J Cutler’s “If I Stay” is really two movies in one. The first is about the blossoming love between a shy cello player and a cool, leather jacket-wearing hunk in a rock band. The second concerns the same cello player Mia (Chloe Grace Moretz) in a coma after a car accident kills her entire family. While in the coma, Mia’s soul can walk around out of body, and she must decide whether she wants to stay with her leather jacket-wearing fellow Adam (Jamie Blackley) or pass on to the next world.

While the teen romance angle isn’t very compelling, it’s the supernatural tearjerker element that throws the movie into extreme sappiness and stupidity.

Mia is your typical quiet, good girl who mostly keeps to herself, having only one loyal friend. She’s a classical-music nerd — she has a sticker in her school locker that says, “I Heart Yo Yo Ma” — and is self-conscious about it. However, her musical talent catches the ear — and eye — of older boy Adam.

With his rugged good looks and laid-back attitude, Adam is a typical teen-girl fantasy boyfriend. They fall in love, and everything seems to be going great until…bam! A family drive on a snowy road turns tragic.

Mia is left an orphan and in a coma and, through flashbacks, we’re told by her out-of-body soul of her backstory.

The teen romance flashbacks aren’t nearly as bad as I thought they would be: It’s more that they’re just so flatline and dull. And for a movie about teenagers, the situation feels far too neat and tidy. Mia has a cool boyfriend who sees her for who she is, and she has two hip, ex-rocker parents (played by Mireille Enos and Joshua Leonard) who are very supportive of her and give the perfect advice when it’s needed.

The only conflict there is the usual boyfriend/girlfriend ups and downs — both Mia and Adam want to pursue their musical dreams — and Mia moping around, afraid that’s she’s not good enough for Adam.

The flashbacks feel underwhelming and derivative. Shauna Cross’ screenplay just sort of hums along, hitting the usual beats. The dialogue, for the most part, is poorly written. And, yet, despite these flaws, the flashbacks in “If I Stay” are tolerable, mainly because of Moretz. The 17-year-old “Kick-Ass” actress is clearly above this material but does a surprisingly good job at playing the shy, nerd girl.

However, “If I Stay” runs into its biggest problems during the “Mia in a coma” segment. We’re treated to an endless barrage of teary-eyed friends and family members having one-way conversations with Mia’s comatose body, while Mia’s soul watches the action. If there was any kind of subtly employed in the flashbacks, none can be found here. And Cutler moves this section along at a snail’s pace, prolonging the inevitable ending.

With a better script and more energetic direction, the flashback scenes in “If I Stay” could have actually made a decent teen-romance picture by themselves. Moretz is good here, as are as most of the supporting cast. Even Blackley isn’t unwatchable.

Fans of the book and the genre will be satisfied. For the rest of us, though, “If I Stay” is yet another lackluster film to come out in this mostly lackluster movie month.