Just to get this out of the way, the plot motivator- the MacGuffin, if you will- is absolutely preposterous. But the execution of the plot is s0 suspenseful and riveting that you just shrug off this improbability- when you learn of it- and go with it. Having said that, on with the plot.
Elijah Wood stars as Tom Selznik, a gifted concert pianist who was on his way to the top when, five years before, he choked during a concert, freezing while playing a piece. He's been unable to perform on stage since then but is now attempting to make a comeback in a concert arranged by his wife Emma (Kerry Bishe). Emma is a singer/actress whose career has been taking off; she loves Tom but realises that he is starting to resent the fact that her career is prospering while his flounders... shades of A Star Is Born.
As I mentioned, the ultimate resolution to Grand Piano is ludicrous but it plays out in a riveting way. A couple of reviews I saw referred to it as "Hitchcockian" which is fitting because apparently the writer Damien Chazelle, who also wrote and directed Whiplash, got the idea from the scene in The Man Who Knew Too Much, where the crash of a cymbal is used to disguise the shot of an assassin's gun. Grand Piano isn't a perfect movie by any means, but it's a fascinating ride while it lasts. Elijah Wood does tortured genius very well- it's something about his eyes. He's also a convincing pianist, because he actually plays; he said that it was very difficult learning to play the music while reciting the tense, fast dialogue. For most of the film, John Cusack is a disembodied voice but he nevertheless manages to project a menace not suprising to anyone who's witnessed his rather unhinged and malignant online presence over the last few years (he's a vocal anti-Israel, pro-BDS, socialist). But a talented actor when he wants to be.
The ending of the film is rushed, perhaps because they realised it was so unbelievable that it was best to get through it quickly before anyone could think too much about it, but I forgive it because I enjoyed the rest of the film so much. It's definitely worth seeking out.