A brilliant, intelligent older woman, highly esteemed for her myriad accomplishments and the quality of her mind, descends helplessly into the childlike forgetfulness of Alzheimer’s disease. Her husband stands loyally by her and tries to tend her at home, but sometimes feels the frustration typical to such caregivers. Ultimately he’s forced to send her to a nursing home, where she passes away in relative peace and comfort.

This is a tale that can (and indeed has) served as the basis for many television-quality movies, and the fact that in “Iris,” the story of the aging husband and wife is periodically interrupted by flashbacks to the couple’s courtship which portray the woman’s vibrant, ebullient youthful persona doesn’t affect its formulaic character. In the present instance, however, the narrative is based not merely, as the phrase usually goes, “on a true story,” but on the actual decline of prolific philosopher-novelist Iris Murdoch, who died in 1999. Screenwriters Richard Eyre and Charles Wood based their script on the published reminiscences of John Bayley, who as a widower has written extensively on his late wife’s life and illness.

Bayley’s works have not been somewhat controversial, but what matters here is that they provide a good skeleton for an effective, if admittedly predictable, depiction of the horrible effects of a debilitating disease. What sets “Iris” apart, raising it to a higher level than it could otherwise have aspired to, is the quality of its acting. Judi Dench is quite extraordinary as Murdoch, capturing the crystalline lucidity of her life as writer and commentator and then portraying, with a remarkable precision that never descends into mere technique, the reality of her painful loss of memory and self-control. A great and gifted actress, she makes Iris real, but never allows her to become merely pathetic; the hint of steel that always remains keeps the performance from slipping into bathos, as would have happened in lesser hands. Jim Broadbent matches her with a superbly shaded turn as the elder Bayley, maintaining the man’s essential gentleness and concern without making him a caricature (and insuring that his occasional emotional outbursts will be truly plausible). Kate Winslet isn’t in quite the same league as the younger Murdoch; she offers an exuberance that seems generalized rather than individual, and never manages to persuade us of the girl’s incipuent genius. On the other hand, Hugh Bonneville is spectacularly good as the young Bayley, matching his characterization so closely with Broadbent’s that the two men actually seem a single person at different points in life.

“Iris” is discreetly directed by Eyre, who moves things along with a sense of decorum that seems quintessentially British; he’s wise enough to leave matters largely in the hands of his fine cast. The picture looks fine, with excellent cinematography and production design; unhappily the score by James Horner isn’t very memorable, despite the fact that violinist Joshua Bell has been enlisted to play the solos it offers.

It’s true that “Iris” doesn’t do a particularly good job of portraying the unique quality of its subject’s work, and the inevitability of its dramatic trajectory is undeniable. But though “Iris” isn’t a great film, it’s a very good one which showcases some great acting: the performances turn into cinematic poetry material that could easily have been prosaic. That should be enough for anybody.