The answer is: nothing. The worst one can say of Quartet is that it’s slight.
Harwood’s play, now transferred to film, was inspired by a real-life retirement home in England for musicians and opera singers, many of whom appear in the picture. Smith plays an imperious diva who arrives on the scene, unaware that three of her erstwhile colleagues are living there—including dithery Pauline Collins, devilish Billy Connolly, and her ex-husband (Tom Courtenay), who still hasn’t recovered from their breakup years ago. The estimable Michael Gambon appears in a supporting role as a fellow resident who is a self-appointed (and self-important) group leader.
If you enjoy these performers you can’t go far wrong with Quartet.