Middle-aged widow Beatrice Hunsdorfer and her daughters Ruth and Matilda are struggling to survive in a society they barely understand. Beatrice dreams of opening an elegant tea room but does not have the wherewithal to achieve her lofty goal. Epileptic Ruth is a rebellious adolescent, while shy but highly intelligent and idealistic Matilda seeks solace in her pets and school projects, including one which gives the film its title. Matilda's science experiment is designed to show how small amounts of gamma radiation from cobalt-60 affect marigolds; some die, but others transform into strange but beautiful mutations completely unlike the original plants. Similarly, Matilda has managed to muddle through a grim existence in a dilapidated, debris-ridden house in a lower middle class neighborhood, learning to deal with her embarrassing mother while managing to avoid becoming anything like her, a future for which her sister seems fated.
of The New York Times observed, "The Effect of Gamma Rays is not a stupid film. The talents of everyone connected with it are unmistakable, including those of Mr. Newman, a director of plain, straight style. It's just that the basic material calls for a kind of second-rate bravura performance from everyone, from the production designer to the actors. There's no way to underplay it. At times I had the feeling that Miss Woodward was auditioning for the role of Sadie Thompson, and that Miss Wallach was occasionally in competition. Only Nell Potts is allowed to perform at something like a reasonable pace. It's a lovely, solemn performance in a film that otherwise succeeds in being simultaneously too barren and too busy, like Beatrice herself." Roger Ebert of the Chicago Sun-Times said the film "is hard-edged enough to be less depressing than it sounds" and noted "Joanne Woodward's performance is not like anything she's ever done before... It serves notice that she is capable of experimenting with roles that are against type and making them work." He added, "Paul Newman's direction is unobtrusive; he directs as we expect an actor might, looking for the dramatic content of a scene rather than its visual style... And the performance by Nell Potts is extraordinary. She glows." Variety commented, "Newman has gotten it all together here as a director, letting the story and the players unfold with simplicity, restraint and discernment." Time Out London called the film "an engaging adaptation" of the play "which sees Paul Newman's cool, lucid direction transforming what could have been a pretentious domestic drama into a touching account of small joys in sad and stunted lives ... Potts steals the movie, but what makes it so watchable is Newman's reluctance to sentimentalise."
