The San Francisco-based millionaire industrialist Walter Williams (Brian Donlevy) has a younger wife, Irene (Helen Walker), who is trying to kill him with the help of her young lover, Jim Torrance (Tony Barrett). After Walter and Irene make plans to drive to Lake Tahoe, Irene feigns illness and asks Walter to instead give Torrance, who is pretending to be Irene's "cousin" from Illinois, a lift to Denver, allowing Torrance a chance to murder Walter en route. The plan falls apart when Williams survives a hit on the head from the would-be killer. Attempting to flee the scene in Williams' Packard Clipper convertible, Torrance dies in a fiery head-on collision with a gasoline tanker truck. The body of Torrance is mistakenly identified as Williams. In the meantime, Irene has made reservations at a hotel in Oakland for her and her boyfriend to meet afterward, under the assumed names of "Mr. & Mrs. Jack Burns".
The wounded, dazed Williams falls asleep in the back of a moving van and ends up in the small town of Larkspur, Idaho. Using the name "Bill Walker", he gets a job as a service station mechanic and falls in love with Marsha Peters (Ella Raines), a young widow who is the station's owner. Meanwhile, the police arrest Williams' wife for his "murder". After Marsha eventually persuades Walter to go back to clear his wife, he is charged with murdering Torrence. Marsha enlists the help of kindly police detective Quincy (Charles Coburn) to prove Walter's innocence.
The "Bayview Apartments" - the site of the Williams' Nob Hill penthouse - in actuality is Weeks and Day's historic Brocklebank Apartments at 1000 Mason Street in San Francisco. The actual town of Larkspur, California was used for filming the fictional town of Larkspur, Idaho. Several areas in and around Larkspur can be seen, including the Probert family's home and gas station at 234 and 238 Magnolia Avenue, the location that was recently occupied by The Tavern at Lark Creek.
This was Anna May Wong's first screen appearance since 1942. Character actor Tom Greenway made his first appearance on screen as an unnamed moving van driver. Gossip reporter Sheilah Graham appears as herself, reading a news item about the case on the radio.
In the 1940s, it was still uncommon for brand name products to be seen in movies, but this was a notable exception. A Bekins moving van is prominent in several scenes. The movie trade paper Harrison's Reports typically called attention to cases in which such products appeared on screen, and always took a stand against that practice. Although its review did not mention Bekins, the Harrison’s review noted "advertising plugs worked in for such products as Pabst Blue Ribbon beer, Raleigh cigarettes, Coca-Cola, Mission Orange soda pop, Mobil gasoline, oil and tires, Gruen watches, and the trade name Rexall."
In addition, Laykin et Cie (of I. Magnin & Co) is featured in the opening credits. Laykin et Cie was a leading West Coast jeweler during the period with an important salon in San Francisco during the time the movie was shot in 1948. In the opening scenes, Donlevy's character Walter Williams presents his wife with a custom Laykin et Cie intertwined diamond double heart brooch with the initials "IW" (for Irene Williams) which was produced for the film. Throughout the film, Irene Williams continues to wear various Laykin et Cie jewels of the period.
At the time of release, the film critic for The New York Times, Bosley Crowther, in reviewing Impact panned the script and plot, writing, "If anyone seeing this picture is willing to string along with that as a fair definition of 'impact,' we can't vouch for the film's appeal to him. For it seems fairly obvious that the authors have geared their intellects to the suppositional level of that phony lexicon. And everything which happens in the picture is as cheaply opportunist and contrived as that arbitrary definition. You either swallow it whole—or you don't. Frankly, your correspondent doesn't."
More recently, critic Gary W. Tooze praised Impact as the quintessential B film: "As far as 'modest' film noirs go, this is one of the best. A simple plot idea is twisted to the max for late 1940s audiences."