The managers of a chemical lab are concerned over their lack of progress in finding a way to fireproof celluloid to satisfy their clients, the Union Celluloid Company, who are applying considerable pressure. Mr Smith, the lab's Chief Chemist, tells the managers that he will have to resign for reasons of health. Of his staff, the longest-serving is Thompson. He asks the secretary Sally to spy on Smith's meeting to find out the outcome. During a tea break, Mary refuses to stop her research, so her colleague John sees that a cup of cocoa is taken to her desk. Thompson tells the rest of the staff that Mary is going to get Smith's job, despite his seniority. All the men in the lab protest against working under Mary, with the exception of John. He believes that she is a very good chemist and is entitled to the promotion.
When Thompson discovers that Smith won't be leaving for another month, he makes plans to take Mary out of the running by distracting her from her work. Thompson decides that John should court her and so ensure that her standard of work drops. John refuses but, under pressure from the other staff, agrees that the winning candidate for the job should be determined by a test. Thompson rigs a test involving a test tube so that John will win. At Thompson's urging, John asks Mary out to dinner. She initially turns him down, but eventually agrees to go and eat a sandwich with him that evening.
At home, Mary's neighbour tells her that some flowers have been anonymously delivered for her. Mary is sceptical but takes one flower, admiring her neighbour's glamorous flat and realising how drab and utilitarian hers is by comparison. While eating, Mary explains to John her theory that society should be organised like that of bees and that men are therefore practically superfluous. John tries to hold her hand, but Mary rejects his advances.
Thompson plots John's progress with Mary on a chart and is dismayed by his poor results. Thompson sends Mary flowers and a love letter in John's name. She comes home after shopping for new clothes and is overwhelmed by the gift. Mary asks her friendly neighbour to help give her a makeover. Thompson brings Sally to John's apartment so that she can give him lessons in seduction. By the time Mary invites him out to dinner, Sally is exhausted by John's new-found passion. John goes to Mary's apartment and is amazed by her transformation and by her beautiful clothes and make-up. They go out to dinner and go for a taxi ride round the park. They fall in love.
John tells the staff that the scheme is over and throws away the chart, but later Thompson retrieves it. Mary comes to work with her new sophisticated look and impresses everybody. Smith leaves and Mary is promoted. Thompson gives Mary the chart to spoil her romance with John and put her off balance again. Mary is furious and shuns John.
Thompson gets the staff to undermine Mary's work by sabotaging the results. John refuses to take part and his experiment is destroyed in an explosion. John is knocked over and pretends to be hurt to get Mary's sympathy. Thompson tells her that John is shamming, so she fires him. Returning home, John throws in the fire the celluloid doll he had been working on. When it doesn't burn, he realises that he has finally found the formula. Breaking into the lab that night, he leaves the formula as a present for Mary. He is caught by the night watchman, who locks him up in the boiler room.
The next morning everyone notices that the heating is not working. Thompson finds John's formula in Mary's office and pretends that it is his own. John overhears Thompson's boasts through the heating duct and is able to show it is a lie as he signed the other side of the paper with the formula. The boiler man and the rest of the lab go to let John out. John and Mary embrace as the heat is finally turned on.
