It had started in his first year, his first transfiguration class, to be exact.
Harry had wanted to get to leave for their class early, to make sure they got there on time, Ron had complained but eventually relented.
They'd walked into the classroom that was marked on their timetables for the class, only to notice there was no teacher, instead, there was a tabby cat perched on the desk, watching them like a hawk.
Harry and Ron shuffled over to sit at a desk, talking quietly to themselves as they waited for the lesson to begin.
When the time passed for the lesson to begin and the teacher didn't appear, they'd all began to get loud as they voiced their confusion.
When five minutes had passed, Dean, one of their roommates, had rushed in, seemingly relaxing when he saw there was no teacher in sight.
And then the cat had transformed into Professor McGonagall.
Harry decided, then and there, that he wanted to learn how to do that, because if he could do that, then he could spend the summer living as a cat so that he wouldn't have to deal with his relatives.
In that year, along with the regular first year course and the issues that came with Quirrell and the Philosopher's stone, Harry learnt everything he could about what he'd learnt was called the 'Animagus Transformation'.
Anyone could do it, so long as they put the time and effort into it, and what you transformed into was based on a few factors, including external issues, energy influxes, and your personality.
Harry wondered what he would be.
When he went to Diagon Alley that summer, he picked up a few books on the Animagus Transformation, looking for information that he couldn't find in school, along with a booklet with information on high quality owl order potioneers.
The Animagus Transformation required a potion, after all, and Harry was not good with those.
In his second year, he opened up an owl correspondence with a potion's master located in Spain, he gave the man an alias so that it wouldn't get out that Harry Potter was looking into becoming an Animagus, not only illegally, but very underaged too.
The man luckily did not care about the fact that what seemed to be a student was inquiring about the potion, after all, said student was willing to pay for it, he certainly had the money.
By the end of his second year, along with killing a thousand year old Basilisk, Harry had a general idea of what he'd need to do when he got the potion.
He would receive the potion on the night of the full moon, once he had it, he would need to add one mandrake leaf that had been in his mouth for one month, re-stopper the potion, and shake it.
Then, after drinking the potion, he should change to his Animagus form.
Simple enough.
In that summer, he managed to acquire the number of galleons that the Spanish potioneer would charge him.
He knew that there wouldn't be the chance to do the transformation while in school, as he highly doubted his ability to hide the fact that he was holding a mandrake leaf in his mouth from the teachers, who likely knew how to spot something like that happening.
So, he would have to do it in the summer while with his relatives who wouldn't care if he didn't exactly speak to them for a month or so, hell, they'd probably encourage it.
He couldn't do it that summer, however, as it was almost over as it was, so he'd need to wait another year, something which didn't exactly bother him too badly, he had plenty of patience from his earlier years.
That year, Sirius Black broke out of Azkaban and Dementors were stationed around the school.
That year, Hermione Granger, a close friend of Harry's was entrusted with a Time Turner.
That year, Harry Potter time travelled.
And when Harry put the mandrake leaf in his mouth that summer, just a couple of weeks later, energy from the Time Vortex still clung to him.
And that had made quite the difference.