In “Iris,” the British novelist Iris Murdoch herself says that education and freedom of the mind are the most important things. Later, when asked about her love for words, she says, “If one doesn’t have words how does one think?”

These are tragic notions for a tragic movie, because we know Iris is destined to be stricken with Alzehimer’s, which will destroy the very mind she lives in. If you don’t know this walking in, you know it soon enough: The film barely lets us meet Fully Functioning Iris before turning her into Alzheimer’s Iris.

That is the film’s greatest flaw. We catch a glimpse of how brilliant and lively Iris was, played with typical classy perfection by Judi Dench. As usual with Dench’s characters, we wish we could meet this character and be her friend. And then, almost immediately, she begins to forget things, to fumble for words and to slip into incoherency. It should not need explaining that incoherent characters do not make for very good protagonists, but that’s what we have in “Iris,” which was directed by Richard Eyre and written by him and Charles Wood, adapted from Iris’ husband John Bayley’s memoirs.

Iris’ descent is intercut with scenes from her early life, in which she is played by Kate Winslet. She meets John (Hugh Bonneville), a stammering young student who can’t believe anyone as gorgeous and popular and Iris would want anything to do with him. He will spend the rest of his life devoted to her, putting up with her occasionally difficult behavior because he knows, at the end of the day, that she loves and needs him.

It is the needing him that eventually becomes centerpiece to their relationship. The older John is played by Jim Broadbent, and this, in fact, is the performance to watch. Winslet, Bonneville and Dench are all fine in their various capacities, but they don’t get to do much. Broadbent, on the other hand, plays a character with flesh and bones, with passions, ideas and thoughts. Most of all, John is filled with sweet, doddering old love for his Iris. The film is about the power of the mind and the tragedy of losing it, but it is also a very moving love story, played with heartbreaking sincerity by Broadbent. He deserves the Oscar for Best Supporting Actor; I only wish the character he’s supporting were given more humanity. Knowing the original Iris would have made her fate even sadder.

B- (; R, because the young Iris likes to skinny-dip;.)