Nick and Nora Charles return from vacation to their home in San Francisco on New Year's Eve, where Nora's stuffy family expect the couple to join them for a formal dinner. Nick is despised by Nora's Aunt Katherine, the family matriarch, as his immigrant heritage and experience as a "flat foot" are considered below Nora. The true reason for their invitation is that Nora's cousin Selma's ne'er-do-well husband Robert has been missing. Nick is coerced into a little quiet detective work for the family.
They easily find Robert at a Chinese nightclub, where he's been conducting an affair with Polly, the star performer. Robert tries to extort money from Selma's unrequited love, David Graham (James Stewart): $25,000 and Robert will leave Selma alone permanently. Unknown to Robert, Polly and the nightclub's owner, Dancer, plan to grift the money and dispose of him. After being paid off, and returning home for some clothes, Robert is shot at the stroke of midnight. David finds Selma standing over Robert and hurriedly disposes of her gun. Despite this, the police determine that she's the prime suspect, and her fragile mental state only strengthens the case. Selma insists that she never fired her gun, and Nick is now obliged to investigate and determine the true murderer.
As suspects pile up, schemes and double-crosses are found and two more murders occur, including Polly's brutal brother. Lt. Abrams (Sam Levene, making his series debut) readily accepts Nick's assistance. Nick follows a trail of clues that lead him to the apartment of a mysterious "Anderson". As in the previous film, the film climaxes with a final interrogation and denouement featuring all the suspects. The murderer is revealed to be David (the mysterious "Anderson"), who has harbored a vengeful hatred of Selma after she passed him over to marry Robert. The case solved, and once again traveling by train, Nora reveals to Nick that they are expecting a baby, although Nick has to be prodded into putting the "clues" together and she comments: "And you call yourself a detective."
Why  does Anderson hate  Selma?
She passed him over to marry Robert