***SPOILERS*** Very much like the 1978 women's movie "An Unmarried Woman" released six years later even down to it's dramatic and unexpected freeze frame ending "One is a Lonely Number" has to do with a young womens abandonment by her husband and being forced to go out in a world and face situations that she's lest prepared for.
Right out of the blue English collage professor James Brower, Paul Jenkins, tells his startled young wife Aimee, Trish Van Devere, that he's divorcing her. The reason for James leaving Aimee is mental cruelty which was in her, thinking it's a piece of junk, throwing out a his rare 1st edition copy of "Paradise Lost" by John Milton! It later turns out that James left Aimee for totally different reasons; His affair with his cute and blond teenage secretary at the collage he teaches in.
Out in the cold with bills piling up to the ceiling Aimee gets a job as a lifeguard at a local community swimming pool in San Francisco. Not at first realizing what she got involved in with the sneering and hot in the pants employment agent Sherman Cooke, Jonathan Lippe, Aimee is later shocked to find Sherman in the ladies shower, fully clothes, demanding that she take a cold shower together with him! It's as if by just doing his job, finding a job for Aimee, Sherman want's to bed her down as a reward! Threatened by an outraged Aimee to have him reported to the police for attempted rape as well as being spotted in the ladies shower by two unexpected visitors, a woman and her young daughter, Sherman makes a hasty retreat never to be seen again in the film.
Aimee eventually gets her head together with the help of fellow divorcées Gert Meredith, Janet Leigh, and Madge Frazier, Jane Elliot, who've had experience in failed marriages with Madge going through her first and Gert her fifth divorce. Aimee also gets the very needed advice and attention that she so desperately seeking from a very unexpected source; The kind and friendly fruit & vegetable man Joseph Provo, Melvyn Douglas. It was Mr Provo, a widower after 39 years of being happily married, who set Aimee straight to what life, as well as good and cheap grade "A" #1 produce, is all about.
Aimee does find out that not every man who's interested in her sexually and romantically is also interested in marrying her. This comes as a big surprise to Aimee when her new boyfriend Howard "The Duck" Carpenter, Monte Markham, reveals to her after a roll in the hay, or the sack, that he's actually married and happily at that! Becoming more and more independent and confident in herself Aimee eventually loses interest in her ex-husband James who's affair with his 19 year old secretary had already gone bust.
This leads James, now feeling like a first class jerk, to drop his filing for divorce against Aimee wanting her back despite all the damage he caused her! But by then Aimee being a free soul with all the eligible and married men in town wanting to date and go out with her she tells James to go take a hike and get himself lost in the woods!
Now free and full of confidence in both herself and her ability to overcome any obstacle put in her path Aimee now confronts the challenge that's been haunting her all throughout the movie! And Aimee, true to form, passes it with flying colors!