Kaleidoscope Century is a science fiction novel by John Barnes. First published in 1995, it is part of the author's Century Next Door series.

The narrator, at first appearing to be thirty-five, wakes up May 27, 2109 in an apartment on human-settled Mars. With no memory of his past, he goes to his werp, a voice-activated laptop computer, and learns that his name is Joshua Ali Quare and that he was born in 1968. From this frame story and a box containing several objects from his past, Quare pieces together what he believes is true about his life leading to the early twenty-second century.

Joshua ran away from home early in his teens to escape his abusive father; while he stayed in an upstairs apartment at Gwenny's Diner, Joshua's mother, a Communist Party member, surreptitiously helped him. He entered the Army at the behest of some Party organizers, and he was put in contact with a KGB operative who provides him with an injection to keep him from receiving or transmitting AIDS, enhance his memory, and periodically regenerate his body, becoming younger with each regeneration and complementary recent memory loss.

Joshua's US Army career is spent in Operation Desert Storm (the First Oil War in the novel) and the "Second Oil War" which culminated in a march on Tehran. After the collapse of the Soviet Union, the KGB diversified and became "the Organization," a counter-insurgency group that supplied technical and logistic support to every side in the Eurowar, a NATO-Central European Union-Russian conflict in the early 21st century. Among the innovations of the Eurowar were Simulation Modeling Optimizing Targeters (SMOTs), advanced smart weapons that attacked an enemy country's natural resources and means of production. As a soldier in the war, Joshua raped a woman and killed her and her family. After the Eurowar ended, Joshua took in "Alice", a war orphan from Prague in a incident where he got the dog tag -- and inspiration for future alias -- "John Childs".

In the 2010s, Joshua worked in Quito, Ecuador on the GeoSync Cable and saw with Alice the beginning of the development of memes that would unify all countries and religions, leading to the War of the Memes (referred to in some of Barnes' other books) that culminated in the takeover of Earth by One True. By the time One True consolidated its hold on the people of Earth, Mars, the Jupiter and Saturn systems had been colonized for decades. Joshua steals another person's ID and becomes an ecoprospector on Mars.

The best scientists and engineers of free humanity had developed the technology to unleash a singularity at the edge of the Solar System that would provide a return point in time and space for the descendants of five transfer ships sent to colonize other nearby star systems. When Joshua finally ventures forth to meet his Organization contact in Red Sands City, he's confronted by a tremendous hustle and bustle of people preparing for the transfer ship descendants to arrive from the 25th century and either destroy One True (and the population on Earth under its control) or confine it there.

A fellow Organization agent named Sadi, who's undergone the same memory enhancement and bodily regeneration process Joshua has, meets him as a woman, his gender of choice after the Organization had perfected the process. From Sadi, whom he'd met as a woman after the Eurowar and also knew as a man, Joshua learns that it's possible to go through the singularity to 1988, when the technology to construct it was first built and put into orbit by the Soviets. By the time of the novel's frame story, Sadi has done this thirty times, each time changing history to his/her benefit. After a time as lovers, Joshua, who's repeatedly refused to accompany Sadi on these excursions through the singularity (also known as a closed timelike curve), is sent by her on a preset course through the singularity once more. Joshua makes plans when he comes back to the late 20th century to change history himself.

However, Sadi also claims he's done this before, and he doesn't remember it. That's why he records everything in his werp and keeps the particular objects he does.