After a lifetime of playing it safe, CalTech professor Robert Godwin, widower, is frustrated and unfulfilled. Hired as a museum consultant to examine a mysterious device found in an ancient Greek shipwreck, he learns something that will allow him to travel through time. Deciding to risk everything, he seeks to change the world by preventing Lee Harvey Oswald from killing President Kennedy. Arriving in 1963, however, Godwin is shocked to discover that he is again a teenager - the same age as the first time he lived through 1963. Determined nevertheless to save Kennedy, he mails letters...