The narrative is non-linear, and begins on a beach in the French Riviera in 1925, but the first act of Tender is the Night spans from just before the death of the mother of incredibly wealthy and beautiful and smart Nicole Diver (when she was 11or 12), to Nicole’s breakdown in the bathroom of her Paris suite in the summer of 1925 (when she is 23 and her brilliant psychiatrist husband, Dick Diver, is 34).
The second act begins when a heartsick Dick and heavily sedated Nicole traveling all summer before returning to their Riviera home. They then move back to the psychiatric clinic where Nicole and Dick met. It probably ends when Nicole grabs the wheel while Dick is driving and tries to kill her family by driving their car off a cliff.
In the third act, Dick and his movie star lover, Rosemary Hoyt, finally, ahem, make it; Dick gets beat all to hell in Rome by the cops; Nicole and her warrior lover Tommy Barban finally make it, too. Dick gets replaced by Tommy, moves back to the U.S., struggles, and practices general medicine, or so we know from the little we hear of the letters he writes to Nicole, who always loves him, and always tries to keep in touch.