The title is a reference to the famous "To be, or not to be" soliloquy in William Shakespeare's Hamlet. To Be or Not to Be was released two months after actress Carole Lombard was killed in an airplane crash.
The action then shifts to later that night. While rehearsing the new play, the theater company has also been performing Shakespeare's "Hamlet", with Maria as Ophelia and Tura in the title role. Bronski commiserates with his friend and colleague, Greenberg, about always being the ones to "carry the spear," instead of having starring roles (pretty much everyone except Tura himself realises he is a truly terrible actor). Greenberg reveals it has always been his dream to perform as Shylock, especially the famous "Hath not a Jew eyes?..." speech. Meanwhile, Maria is inspecting a bouquet of flowers she has received. Her maid and dresser, Anna, wryly comments that this is the fourth bunch in just as many days, and Maria reveals that they come from a young pilot who has been at all those four performances. Tura comes in and is immediately jealous and suspicious about the flowers, but Maria consoles him, as she always does, by telling him what a wonderful actor he is. He then leaves the room. Shortly afterwards the handsome, young pilot asks on a note delivered by messenger for permission to finally meet Maria. She pretends to debate whether to grant this request, but of course does, writing to tell him in response that he should come to her dressing room when Hamlet (i.e. her husband) begins his "To be or not to be..." speech, so they can be sure of privacy. Tura's reaction is hilarious when, just as he begins to speak, a young man (i. e. the young pilot) from one of the front rows stands up and edges his way out. This young man turns out to be Lt. Stanislav Sobinski, a pilot with the Polish Air Force, who has been in love with Maria for quite some time, and has seen her in most of her productions. Maria is very attracted to him, but clearly sees him only as an opportunity for a passing fling. Stanislav questions her about various comments she has made in magazines, revealing that the lieutenant has a rather unhealthy obsession with his idol, and that Maria has not been entirely truthful about her life circumstances in interviews. In the end, Stanislav offers to take Maria up in his plane the next day, and she agrees. No sooner has Sobinski left than Tura returns, catatonic, shocked that someone has actually walked out on him. Maria suggests the person might have been having a heart attack and had to leave, and may even be dead by now. Tura is greatly comforted by this.
A few days later, as the company listens in disgust to one of Hitler's speeches over the radio, a representative from the Polish government arrives with an order to cease production of the Gestapo play, for fear it may "offend Hitler". The company expresses their disdain for this, but are forced to perform Hamlet yet another night. Once again, Lt. Sobinski walks out during Tura's speech (this has clearly become his and Maria's "thing"). He meets up with Maria backstage, and, much more confident than before, asks her if she "likes" him. When she assents, he rejoices, assuming this means she will leave her husband, give up acting, and come live with him on a farm. Before she can figure out how to explain this is not what she wants, Anna the maid rushes in, saying Germany has declared war on Poland. Stanislav and Maria embrace, wondering if this is the last time they will see each other. The actors take shelter under the theater as bombs begin to fall.
A montage and voice-over show us Hitler conquering Poland, panning over the destroyed signs of the shops we saw at the beginning of the film. It also tells us that the Polish resistance has been sabotaging the new Nazi regime, and that the Polish division of England's Royal Air Force is doing its best to free its mother country. We cut to this very division, where a group of young pilots, including Lt. Sobinski, are singing a rousing song with a certain Professor Siletsky. Siletsky is a part of the resistance movement, and intimates that he will be returning to Warsaw soon. Joyous at the thought of contacting their families, the pilots write down addresses for the professor to visit. Sobinski himself asks Siletsky to give Maria the message "To be or not to be", but his suspicions are aroused when Siletsky, who claims to have lived his whole life in Warsaw, does not know who the famous Maria Tura is. After the Professor departs, Stanislav goes to his superior officers, who are equally disturbed, although more that Siletsky revealed his secret assignment to Warsaw, and that, even if he himself is not a Nazi, he is carrying the addresses of the pilots' vulnerable families very near the wrong hands. They decide to have Stanislav fly to Warsaw in the hope of reaching it before Siletsky and warning the resistance about him, by placing the professor's photo in a certain book at a certain bookshop. Stanislav parachutes in, but is chased by soldiers and dogs, and cannot get near enough to the bookstore. He runs away. Time passes, and Maria Tura appears, and performs the task without arousing any suspicion. As she is returning to her small apartment, where Stanislav lies asleep, she is stopped by two Nazi soldiers, who say that Professor Siltesky has asked them to bring her to him at his hotel. She has no choice but to follow. Siletsky appears and delivers the message "To be or not to be" (believing it is a spy code), whereupon Maria innocently tells him what it really means. Reassured, he sets about trying to seduce her in the hope of converting her into a spy for the Nazis. She pretends both to follow along and be attracted to him, and, when he invites her to dinner that night, finds a way out of the room by saying she needs to change into something more appropriate. He lets her go and she rushes home.
Meanwhile, before she arrives, Tura has come home to find Stanislav asleep in his bed. Hiding in a corner, Tura proclaims, "To be or not to be," whereupon the pilot sleepily gets out of bed (it has by now turned into something if a reflex). Tura realizes who this is and confronts him. Before Stanislav can explain, Maria runs in. A three-way conversation occurs in which Maria fills Stanislav in on events, they try and figure out what to do (killing Siletsky is the only option, they conclude), and Tura tries to figure out what on Earth is going on. In the end, Tura proclaims that he will kill Siletsky for him, but only as long as they then agree to tell him why he has to.
Later that evening, Mrs. Tura returns to the professor's room in a beautiful gown. He is charmed. She manages to get him to sign his name on a blank piece of paper, telling him she can divine what kind of man he is from his signature. She says he is good, strong, and mysterious, and they kiss just as there is a knock at the door. It is a Nazi officer (who we recognize as actually one of the members of the acting company). He informs the professor that he is wanted at Gestapo headquarters.
The film was nominated for an Academy Award for Best Music, Scoring of a Dramatic or Comedy Picture. In 1996, it was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically, or aesthetically significant".
A stage version written by Nick Whitby is scheduled for a Broadway production by the Manhattan Theatre Club, New York City, at the Samuel J. Friedman Theatre, starting previews on September 16, 2008 and opening on October 14. The director is Casey Nicholaw, with principal cast including Peter Benson, David Rasche, Peter Maloney, and Jan Maxwell.
A prescient line was cut out of the film after the death of Carole Lombard: when Lombard is invited by Robert Stack's smitten airman to fly in a plane with him, she says: "What can happen on a plane?" The line has since been restored to available prints of the film.