Deep Impact opens to a starry night sky in Richmond, Virginia. The camera slowly pans down where stargazers and amateur astronomers are gathered with telescopes. Among these are students and faculty of the Lee High School Astronomy Club. Two students at this club are Leo Beiderman (Elijah Wood) and his girlfriend, Sarah Hotchner (Leelee Sobieski). As they look through their telescopes at the stars, Leo innocently asks Sarah about another male student who invited her to a student party.
Leo and Sarah's teacher, Mr. Perry (Mike O'Malley) comes up and quizzes them about some of the brighter stars. Leo correctly identifies the stars Mizar and Alcor, but cannot name a somewhat smaller, dimmer star a bit south of the other two, although he is sure that it is not the star Megrez, as Sarah claims. Mr. Perry, thinking it might be a satellite, suggests they take a photograph and send it to Dr. Wolf, a professional astronomer who sponsors the Lee High School Astronomy club. As Perry walks away, Leo and Sarah get into a brief, teasing argument about whether the unidentified star is Megrez.
Dr. Marcus Wolf (Charles Martin Smith) arrives for work at the Adrian Peak observatory in Tuscon, Arizona. Seating himself at his work station, he finds the packet sent him by Leo, containing a photograph of the unknown star near Mizar and Alcor, and a cover letter asking him to identify it. Punching the stellar coordinates into his computer, he sees it is an uncharted object.
His observatory's telescopes zooming in on it, he sees it is in fact a comet. Curious about it, he directs his computer to analyze the comet's position and calculate its trajectory. As the numbers begin to scroll through the screen, Dr. Wolf's curious expression fades into a look of severe worry and concern. An orbital interpolation graphic on his monitor overlays the comet's projected path on a small display of the solar system-- and a large dot quickly appears on the third elliptical line around the sun's icon; the comet is projected to strike Earth.
Seizing a floppy disk, Wolf saves all the data to it while instructing his computer to open a UNIX mail server to send the information out in email. After over a minute, however, he only gets two responses that the server is down. Exasperated and in a desperate hurry, Dr. Wolf grabs the floppy and stuffs it, along with the packet received from Leo, into a padded manila envelops. He writes his own surname and Leo's surname (Wolf-Beiderman) on the floppy drive and rushes out of the observatory to his jeep.
As he races down the road, he struggles desperately to place a call on his cell phone to the Department of Planetary Sciences. Cell phone technology of the year (1998) being crude and limited, it takes him several long, harrowing minutes just to reach the automated answering system. Meanwhile, an 18-wheel big-rig truck is coming down the road. The driver, smoking a cigarette and listening to country music, is badly sleep-deprived; drinking a high-sugar soft drink to keep himself awake. As Dr. Wolf, pre-occupied with trying to finish placing his call and reaching a live person, focuses more on his phone than on the road, the driver's cigarette slips from his mouth, hitting his lap.
The pain causes him to involuntarily look down while he bats at his lap and the ash from his cigarette. Both Dr. Wolf and the truck driver finally look up, alerted to the shine of each others' headlights, and see they're about to collide. Both desperately try to swerve, but the truck's cab lags behind in the swerve and Dr. Wolf's jeep slams into it, and he's knocked off of the road and down an incline. The envelope flies off of the seat beside Dr. Wolf and out the window of his jeep as it tumbles down the incline and explodes, killing him instantly.
Washington, D.C., one year later. MSNBC computers are transmitting a news report. In a conference room, a small team of news reporters led by Stuart Caley (Bruce Weitz) are discussing the resignation of U.S. Secretary of the Treasury, Alan Rittenhouse (James Cromwell). Rittenhouse is reportedly resigning because his wife is sick, although Caley's team doesn't buy it, going over a number of unpopular decisions recently made by Rittenhouse which has earned him more than enough enemies to force him out of his seat. The team jokes that with all the trouble Rittenhouse has recently gotten into, it's small wonder his wife is sick.
But reporter Jenny Lerner (Téa Leoni) demurs, saying Mrs. Rittenhouse is an alcoholic. She claims to have found out from a connection in the U.S. Treasury named Mike Woodward. As the news team starts discussing an idea on the price paid by wives of governmental figures, Jenny is asked about her connection to Woodward by Beth Stanley (Laura Innes), the news team's White House correspondent and Jenny's direct supervisor on the ladder. Jenny explains that Woodward has a crush on her, and she took advantage of it to get information on Alan Rittenhouse.
At that point, Jenny quickly asks Beth about the possibility of stepping into a recently-vacated anchor position in their late-night weekend news broadcast, but Beth quickly rules Jenny out as a replacement anchor. As the conference adjourns, Jenny tries to press Beth on it, and Beth (a bit impatiently) explains why she feels Jenny needs more time to build herself up before being a weekend news anchor. She asks Jenny to find out more on the Rittenhouse resignation and have any updates on her desk by the next day.
Jenny meets her mother, Robin Lerner (Vanessa Redgrave) for drinks and lunch. Robin is preoccupied with the courtship of her now ex-husband, Jason (Maximilian Schell) to a younger woman named Chloe (Rya Kihlstedt)-- in fact, the age difference is such that Chloe is only two years older than Jenny, despite now legally being her stepmother. Jason and Chloe have very young twin children together and have just married that morning, and Robin prattles on about it.
Carrying on with her assignment, Jenny meets with Patricia Ruiz (Concetta Tomei), longtime former assistant and secretary for Rittenhouse. Ruiz is somewhat bitter because she was completely dismissed from her position without being conveyed to Rittenhouse's successor, because she followed him from his former position as Connecticut governor, not hired from within the federal pool. Ruiz confirms that Rittenhouse cheated on his wife, and she suspects he resigned because he was about to be found out.
Ruiz, therefore, partially blames Rittenhouse for her now being out of a job. She says that Rittenhouse had had a special, secondary private telephone line installed in his office that only he was authorized to answer. But, Ruiz says, one day she overheard a name; a woman's first name she suspects as being one of Rittenhouse's lovers: Ellie. Ruiz also mentions she overheard Rittenhouse speaking about this Ellie in a conversation with the U.S. President himself.
Jenny arrives with an MSNBC news van to speak directly to Rittenhouse. They find him on a small boat with his young adolescent daughter, who is cold and suspicious of the news crew. Rittenhouse curtly tries to stick to his story about his wife being ill, until Jenny drops Ellie's name. Rittenhouse says he will only speak to Jenny if her cameraman turns his camera off.
Once this is done, Jenny reveals what she's found out; more than enough to disprove the claimed reason behind Rittenhouse's resignation. Rittenhouse insists she knows far less than she thinks she does, and tries to appeal to Jenny's humanity, asking if she can understand that he wants to be with his family more.
Driving down the highway by herself now, Jenny speaks into her private voice recorder, making a personal log in which she hypothesizes that the President, not Rittenhouse, was having a love affair with this Ellie, and Rittenhouse took the rap for him to protect the President's own reputation and credibility, and was given significant financial compensation.
Suddenly a black car hits hers from behind, not hard enough to overturn her car but hard enough to jostle her and make her take immediate notice. And this car is not alone-- three others are in formation, moving to surround Jenny's car. The car behind her bumps hers a second time. Scared out of her wits, Jenny realizes these men are intent on forcing her to take the nearest exit off the highway and shouts to the car on her left that she will comply.
The exit's re-merge into another highway is barricaded with a setup of road pylons. Jenny brings her car to a stop and the car in front of hers moves to block her from any further motion. The men in one of the cars pull open Jenny's driver-side door and identify themselves as FBI, ordering her out of her vehicle and into their own. One of them gets into Jenny's car to drive it where they are taking her.