In the deep, dark, no-longer future of 1998, the world is in rough shape. Algae blooms have begun spreading poison across the oceans, millions of people starve in underdeveloped countries, and entire species have begun going extinct. It's not a pretty picture.
But Dr. John Renfrew has an idea: He is going to use tachyons, theoretical particles that can move faster than light, to send messages back in time to warn the past, hopefully stopping the disasters before they start. Renfrew teams up with a plasma physicist named Gregory Markham, and together they convince Ian Peterson to get the National Research Council to fund their experiment. Peterson manages to secure their funding—if only to see if the crazy plan will actually work.
On a trip to California, Peterson discovers a note left for him in a safety deposit box… a note left by someone from the past who has received their messages. Clearly the transmissions are getting through to the past, then, but Peterson, Markham, and Renfrew must question whether they are doing any good. After all, the diatom blooms are still spreading, and the world is descending into ecological chaos.
Markham attempts to mathematically work out how their meddling will affect time, and while he's at it, solve the riddle of time, too. While returning to America on an airplane, Markham reaches a revelation immediately when the plane crashes, killing him and leaving both the reader and the other characters in the dark about exactly what he discovered.
The diatom clouds spread across the world and begin poisoning food everywhere. Peterson gets sick after eating some contaminated food. After recovering in the hospital, he heads to his house, grabs his bug-out bag before, and heads for the hills where he has a fort built to withstand Godzilla and then some.
Renfrew continues to try to send messages back in time, but as the hours mount and his resources dwindle, he grows weary and sickly. Eventually, Renfrew receives a message that might be from the year 2349 right before the power goes out. Without knowing if he succeeded in changing the past, Renfrew leaves his lab and heads home to live out the rest of his life, short as that may be.
In the year 1962, Professor Gordon Bernstein receives interference in an experiment he is working on with a graduate student named Cooper. After trying everything to block the interference, Gordon notices that it is too regular to be a natural interference. Using Morse Code, Gordon and Cooper decipher a message within the interference.
Gordon eventually goes to seek the help of other experts in an attempt to understand the message. An astronomer named Saul Shiffer decides to use the data to propel his own theories about extraterrestrial life, and Gordon receives backlash from the scientific community and his colleagues at La Jolla university because, you know, extra terrestrials? Come on.
The inciting incident for the 1962-63 storyline comes after Gordon presents his colloquium. Ridiculed, laughed at, and all together abandoned by his colleagues, including Lakin, Gordon is humiliated and has every reason to abandon the data that doesn't fit with how reality should work. But the data is real, and Gordon knows it. Time to put up or shut up.
Gordon takes the data back to the beginnings. He goes over every scrap of information and finally decides he can't be wrong. He recruits his old professor, one Claudia Zinnes, to use their equipment to look for the signal, too. Gordon spends days and evenings, night after night in the lab searching for the messages and some clue as to their origin. The work not only strains Gordon's work as a professor, but also his relationship with his girlfriend, Penny.
One by one, the clues come together. Ramsey's studies prove the pesticides mentioned in the messages could exist even though no one has synthesized them, and his lab experiments also prove how super dangerous they could be. Claudia Zinnes's experiments also pick up the messages proving they aren't an accident. Finally, Gordon discovers the truth about the messages and publishes his findings.
In 1963, a boy named Bob Hayes goes to a book depository to look up information on Gordon's research when he hears a gunshot in the floors above. Investigating, he finds Lee Harvey Oswald shooting at the president and stops the would-be assassin from firing his famous third shot. Watching the event on the news, Gordon realizes something of importance has happened that day.
In 1974, Gordon goes to Washington D.C. to receive the Enrico Fermi Price. He thinks over the implications of his work and knows that the timelines split in 1963, forming two different universes, forever disconnecting him from the 1998 timeline. As he accepts his award, Gordon looks over the faces of the crowd and sees the waves of time breaking and crashing through them all.