Doctor Manhattan is Jon Osterman. He is a superhero.
His father was a watchmaker, and Jon planned to follow in his footsteps. When the US drops the atomic bomb on Hiroshima, Jon is sixteen. His father, confronted with the undeniable facts of the theory of relativity, declares his profession outdated and throws his son's watch-making parts out the windows, urging him to instead pursue a career studying nuclear physics. Not only is this the turning point in Jon's potential future from watchmaker to nuclear physicist, it foreshadows Doctor Manhattan's 'exterior' perception of time as predetermined and all things within it as so determined, including Doctor Manhattan's own reactions and emotions.
Jon attends Princeton University from 1948-58 and graduates with a Ph.D. in atomic physics. In early 1959, he moves to a research base at Gila Flats, where experiments are being performed concerning the 'intrinsic fields' of physical objects which, if tampered with, result in their disintegration. Here he meets Janey Slater, a fellow researcher; they are eventually to become lovers.
During a trip to New Jersey in July 1959, Jon and Janey visit an amusement park. There Janey's watchband breaks, and the watch is damaged when a fat man steps on it. Jon decides that he can repair the watch, and tells Janey so. That night they sleep together.
One month later, in August, 1959, shortly after his thirtieth birthday, Jon goes to give Janey the repaired watch, only to discover he has left it in his lab coat, which is inside the intrinsic field experiment test chamber. While Jon is inside the test chamber retrieving his coat the door closes, automatically locking as a safety feature. Unable to open the door or override the countdown, Osterman's colleagues - save for Janey, who cannot bear to see the last moment and flees the room - can only watch, horrified, as the countdown for the current experiment shortly reaches zero, and Jon has his 'intrinsic field' removed. Bathed in the radiant light, he is torn to pieces from the force of the generator, instantly vaporized and officially declared dead.
The following months see a series of strange events and apparitions at the research base, leading residents to speculate the area is now haunted. It becomes plain that Jon has been progressively reforming himself during this time. This progression being indicated by a series of partial bodily reappearances: first as a disembodied nervous system, including the brain and eyes; then as a circulatory system (November 10); then a partially muscled skeleton (November 14). Each time, the appearance only lasts for a few seconds. Jon fully reappears on November 22 as a tall, hairless, naked, blue-skinned man.
After his transformation, Jon begins to experience time in a non-linear, "quantum" fashion, and it is implied that he is aware of and experiencing all the moments of his life simultaneously. Jon is not omniscient; he remains reliant on his intellect to reach conclusions, but his range of sensory data has been abruptly extended, in proportion to the lessening of his emotional capacities. This often leads him to arrive at conclusions greatly different from those available to normal humans. His already weak will (marked by his apparent submission to his father's career plans, whatever they might be) becomes sublimated further during this time. He increasingly has difficulty acting in what those around him consider the present moment, leading to many accusations and even the public perception that he is emotionless and uninterested in human affairs. For instance, he does nothing to prevent the assassination of President John F. Kennedy, even though he is aware it is going to happen as he meets the President. His apparent lack of sentiment is more a matter of radically altered priorities, owing to a colossal, unbridgable gap of perception between Jon and the rest of humanity.
He subscribes to a deterministic view of events. Doctor Manhattan appears to exert an effort of choice, and his actions often seemed governed by a rigidly utilitarian code of ethics in which the correct course of action must be the one that benefits the most. In some sense, unlimited power has come at the cost of the total absence of responsibility, and his growing detachment, if not apathy, is juxtaposed with his apparent ability to do anything.
Jon's presence succeeds in tipping the balance of the Cold War in the West's favour, and the United States consequently becomes more aggressive and adventurist during this period. His abilities also radically alter the world economy, as he can, for example, synthesize the massive amounts of lithium required for all motor vehicles to become electric. At President Richard Nixon's request, he brings America victory in the Vietnam war within three months. This victory distorts the American political process, as the 22nd Amendment is repealed and Nixon is then repeatedly reelected.
During the first meeting of the 'Crimebusters' superhero group, Laurie Juspeczyk, Silk Spectre II, catches his eye. His relationship with Janey Slater ends acrimoniously shortly after, and he begins dating Laurie.
During the execution of Ozymanidias' plot to save the world, Manhattan is accused of giving cancer to those exposed to him over long periods of time. It emerges that this is untrue, for it is rather a careful fabrication of Ozymandias', but this revelation is not quick enough to prevent Manhattan from exiling himself to Mars. Eventually, he brings Laurie (who, in the meantime, has taken Dan Dreiberg/Nite Owl II as a new lover) to Mars, where they argue over the fate of the human race.
Doctor Manhattan is working in the Rockefeller Military Research Center for the U.S. Government. He is living with Silk Spectre II, Laurie Juspeczyk.
He leaves Earth for Mars when he is accused of causing cancer in his close associates over the years. However, this was a frame arranged by Ozymandias to induce Osterman to leave, to remove his interference in his scheme to save the world. Eventually, he brings Laurie to Mars to discuss why he should do anything to aid humanity, an argument Laurie inadvertently wins when she goes through her life and realizes to her shock that her father is the Comedian, a man whom she despised for sexually assaulting her mother. From that revelation, Doctor Manhattan is amazed by the improbable chances that occurred to result in the birth of Laurie, which he sees as a stunning "thermodynamic miracle". By extension, this miracle can apply to any living thing on Earth, and so Doctor Manhattan decides to return to Earth to protect this wonder called life.
Although they return too late to stop Ozymanidas' plan, they teleport to Antarctica to confront him. Ozymandias hinders Doctor Manhattan with a tachyon generator that interferes with Doctor Manhattan's ability to see the future, and then disintegrates him by subtracting his intrinsic field. Doctor Manhattan restores himself much more quickly this time, but when Ozymandias reveals that his scheme appears to have averted a looming nuclear war, Doctor Manhattan realizes that to expose the scheme would be too dangerous for all life on Earth. Doctor Manhattan and the other superheroes except for Rorschach agree to keep quiet to preserve Ozymandias' results. Rorschach leaves on his own and is killed by Doctor Manhattan to prevent him from ever telling the truth. Manhattan does so reluctantly, at Rorschach's own insistence, who asserts that his death is the only thing that will ensure his silence. Doctor Manhattan does not mention Rorscach's death when talking to Ozymandias not long after.
Doctor Manhattan decides to depart Earth again, but he might return one day. Ozymandias is surprised by his decision, pointing out the apparent contradiction with Doctor Manhattan's apparent renewed interest in human life.
He is shown to be immensely powerful and invulnerable to all harm; a clear limit to his powers is never explicitly shown. Jon has complete awareness of and control over atomic and subatomic particles. He can alter his body's size, coloration, density, and strength. He does not appear to age, need food (although he is shown to eat regardless of this), water, or air, and is for all intents and purposes immortal at thirty years old. In addition, he is even able to reconstruct his body if completely disintegrated. He can teleport himself and others over great (even interplanetary) distances, but the exact limit to the number of people other than himself he can teleport at one time or the distance he can teleport them is unknown. It is quite possible there may be none since on one occasion he teleports a whole crowd of rioters away from the streets and each into their own individual homes. However, he seems unable or unwilling to prevent the side effects others experience during teleportation; Laurie, for instance, always throws up on arrival and two demonstrators had heart attacks--something Jon seems to dismiss.
Due to his non-linear perception of time, he can also see the future (and the past), although he is apparently unable to change it, regardless of the outcome. His only weakness appears to be tachyons; a large burst of them can 'jam' or slow his ability to see the future to a large extent, as well as temporarily confusing his perception of the "present time". In addition to these powers, Jon is able to phase any part of his body through solid objects without damaging them, produce multiple copies of himself which function independently of each other, project destructive energy, create force fields, transmute and create matter, move objects without physically touching them (telekinesis), reverse entropy, and, he suggests, create life. In the event of a nuclear war, he would be capable of destroying upwards of 60% of all Soviet nuclear missiles while at the same time 'destroying' large areas of Russia. As a result of these capabilities, Jon becomes central to the United States' Cold War strategy of deterrence.