This is one of my favorite movies of all time. Hepburn, Grant, Stewart. It just doesn't get better.
Written as a stage play for Hepburn and remade as a musical with Sinatra, Crosby and Grace Kelly, it's a witty comedy of manners where Hepburn owns her role.
The story is of a society divorcee (Hepburn), planning to remarry a stick-in-the-mud self-made man. Enter ex-husband Grant, with Spy Magazine reporter Stewart and photographer Ruth Hussey. This is not pleasing to the publicity-disdaining Hepburn, but the magazine editor has some scandalous information about Hepburn's father that he's using as blackmail.
Of course there's the post-divorce sniping between Hepburn and Grant, each only too happy to point out the others' faults to Hussey and Stewart.
To give away too much would spoil it in the end, but I'll say this: Hepburn and Stewart share the single most romantic moment on film. Watch for it, after the pool scene.