Teddy Lupin grew up much like Harry Potter himself had grown up - under the care of those who were not his parents. Remus and Nymphadora Lupin had fallen in battle, leaving behind their infant boy. All things considered, Teddy had a pleasant life. Nothing, however, can make up for the lack of a mother and a father. It was a sense of loss and absence that haunted the poor boy throughout his adolescence, haunted him to the point that he became determined to do something about it. His girlfriend, Victoire, feared that Teddy was growing obsessive and urged him to let the past remain in the past. Teddy would not be deterred. Luck - or fate, as he saw it - smiled on him one day while he rummaged in the Weasley attic while he and Victoire babysat for Ron and Hermione. He'd been looking for old photographs of his parents, but what he found was much, much better. A timeturner, Victoire had explained. Quite suddenly, Teddy had the answer to his problems right there in his hands. He need only turn the blasted thing, help his parents at the battle, and they would grow together as a family. Or - say - he could go back even further, and suppose there needn't be a battle at all? Suppose Voldemort never started a war at all. Suppose Teddy could warn someone - anyone - before Tom Riddle even had a chance to become Lord Voldemort!
Teddy's primary mistake was letting Victoire in on his plan. She argued that he was out of his mind, that time was not something to be tampered with, and begged him to put it back or she'd warn Hermione. Before she'd even managed to get her last word out and before Teddy had really even planned his actions at all, he was turning the device. Victoire, horrified, reached for Teddy and grabbed his wrists. A scuffle ensued, and rather than revisit the year 1998 and help out at the Battle of Hogwarts, Teddy landed himself - and Victoire - in the year 1978. They posed as a young couple by the names of Teddy and Victoria Thyme, hoping to go relatively unnoticed and unchallenged for their presence.
At least, that's what Victoire planned to do. Teddy began seeking out the one known simply as the Dark Lord. Indeed, it did not take the boy long to find him. Voldemort might have shrugged Teddy off as a fly and had him done away with had Teddy not possessed intimate knowledge of Voldemort and the Death Eaters that could not have possibly been coincidence. Voldemort then took a keen interest in this peculiar boy and his young friend, though she did not prove nearly as useful. They were young, and thereby they were easily manipulated - and controlled. They were putty in his hands, and Voldemort - quite like Teddy - had quite suddenly had the answer to his problems right there in his hands. Teddy's intentions may have been to stop Voldemort in his tracks and create a better future for those he loves most, but his inexperience has quite possibly doomed them all.
With this new information gathered from the depths of Teddy and Victoire, Voldemort rose to enormous power. Having heard of the prophecy and of Lily Potter's protective love of Harry Potter, Voldemort never targeted them, never marked Harry as his equal. He never attempted to kill Harry, and he never lost any of his power. The year is now 1982, and Voldemort has taken throne as the Minister of Magic over Wizarding London. His mission is to purify the wizarding community and, ultimately, the world. One of his first projects as Minister of Magic was to require every witch and wizard to register with the Ministry of Magic. Those who refused were sent to Azkaban and rarely heard from again. It didn't take long for muggleborns to begin being rounded up by the Aurors - who were now made up almost entirely of Death Eaters and Death Eater supporters - and killed. Half-bloods and full-bloods were stripped of their jobs and required to turn over their wands to the Ministry. They work in the homes of the pure as servants, much as House Elves do, or in menial jobs. Only those who are pure live comfortably, and that in itself is no longer a given. In 1980, Voldemort proposed a plan that would lead to the purification of the wizarding world, one child at a time. Any marriage between a witch or wizard of pure blood that was not to another witch or wizard of pure blood would be dissolved. Purebloods would be required to marry another pureblood. Voldemort was not inconsiderate, of course. He allowed all purebloods precisely one year to select a suitable mate for him or herself; if one had not been picked in the allotted time, the Ministry would assign a spouse and allow one following year to arrange the marriage. Couples were expected to begin having children within two years of marriage.
Naturally, many of Voldemort's new implements were met with resistance. The Order of the Phoenix exists, but they do not have the power that Voldemort and his cronies possess. They are an underground organization whose primary mission now is to protect. The majority of the Order was and is made up of those witches and wizards the Ministry deems "tainted." Muggleborns and half/fullbloods disappear every day. The Order cannot afford to lose any more of their numbers, nor of their friends. They are fighting to regain their footing in this revolutionized world in the hopes that, one day, they will manage to overcome Voldemort and his Death Eaters.