Scientist Waldo Samson (Alec Craig) has discovered how to manufacture cheap synthetic diamonds that are nearly identical to actual diamonds, as he demonstrates to diamond industry representatives and New York Police Inspector Mike O'Hara (James Gleason). Samson only wishes to provide them for the American defense effort, but O'Hara insists on providing him with a police guard.
Ruthless criminals, however, abduct Samson to gain his secret. O'Hara recruits a reluctant amateur sleuth Gay Lawrence (George Sanders), known as the "Falcon", to search for him. Meanwhile, Helen Reed (Wendy Barrie), the Falcon's fiancée, becomes increasingly frustrated as the crime solving interferes with her marriage plans. Lawrence meets exotic jewel thief Rita Mara (Mona Maris) and suspects she is involved in Sampson's disappearance. Rits pulls a gun on Lawrence, and forces him to accompany her. Helen finds Rita's purse and inside, the address of Rita's hotel.
Lawrence escapes but learns from his sidekick, Jonathon "Goldie" Locke (Allen Jenkins) that Helen has gone to Rita's hotel, looking for him. Learning that Sampson is also registered there, Lawrence breaks into his room and finds the scientist, dead. The police arrive shortly after and attempt to arrest Lawrence, who asks O'Hara for a 12-hour reprieve to find the real murderer.
When Lawrence asks Helen to meet him at a nightclub, he knows follow Helen and Goldie will be followed. At gunpoint, Rita abducts Lawrence once again and takes him to her accomplice in a warehouse where Max Carlson (Victor Kilian) is holding the real Waldo Sampson captive. After obtaining the secret formula, Max decides to kill Sampson and Lawrence. Rita knocks out Lawrence, and turns on her partner, killing him, but does not locate Sampson's formula.
When the police arrive at the warehouse, they find Lawrence. arresting him for murder with O'Hara taking him to headquarters, where Helen has been arrested as Lawrence's accomplice. Rita and the remainder of the gang is brought in, and when Goldie produces the secret formula which he had taken from Max, Lawrence explains that it was Herman, Waldo's twin who was killed in the hotel and that Rita killed Max for the formula.
Lawrence and his fiancée finally are able to resume their romantic getaway, but once on board their aircraft, a beautiful young woman greets Lawrence, arousing Helen's jealousy.
In A Date with the Falcon, the Falcon is engaged to Helen Reed, but leading lady Wendy Barrie would never appear in future sequels. RKO had been trading on the British actress's notoriety as the girlfriend of gangster Bugsy Siegel, but did not have a continuing role for her in mind. A Date with the Falcon would be the first of many sequels that featured other actresses in leading roles. The series also became a breeding ground for other talented studio contractees including director Edward Dmytryk and actors Barbara Hale and Jane Greer. The Falcon series was also the progenitor of a new genre - film noir.
In his review of A Date with the Falcon, Bosley Crowther wrote, in The New York Times, that, a pattern existed in the Falcon series "... the pattern. Mr. Sanders, an idle man of the world, is just about to be married—this time to Wendy Barrie—when a mystifying crisis arises—this time the disappearance of a scientist. Obviously Mr. Sanders doesn't care to enter the case; he never does—or never did, perhaps we should say. But duty and the lure of adventure inevitably drag him in. And so, for fifty or sixty minutes, he is off on a serio-comic chase, sleuthing a gang of murderous smugglers, while his girl and the police act bored and dense."