Conventional wisdom has long held that anytime something comes in a set of three, it's always the middle bit that's most likely to be rubbish. This universal truth can be seen in everything from movie trilogies to children, and eventually led the Hingligee race of the planet K'lorporti to do away with the concept entirely, which worked quite well until it came to the sticky subject of bridges. (After a heated and protracted meeting of the Hingligee parliament, it was eventually decided that rather than renounce the scientific paradigms of the age and add a middle section to the bridges, they would simply ban rivers.)
On the subject of the Hingligee the Hitchhiker's Guide to the Galaxy says nothing at all, because the entire planet of K'lorporti was obliterated eons ago when they left the middle out of their new super-sized particle accelerator and switched it on.
In any case, as the middle is where we now find ourselves the narrator resolves to do their utmost to prevent it going to shit; a predicament mirrored by the subject of our observation, Number Five, who found himself both toward the end of his life and in the middle of it, because time travel is a complicated business that can have any number of bizarre and far-reaching temporal side effects such as becoming one's own grandfather, erasing oneself from the timeline or kick-starting the industrial revolution a hundred years too early by accidentally inventing processed cheese.
Now the reader may remember the side effect of Number Five's own experiments with time travel had him stranded for forty years or so in a post-apocalyptic future devoid of life save for an incorporeal alien presence named D'Olo0rz which had taken up residence in his brain and was subsequently driving him mad while attempting to manipulate quantum physics enough to get them both home. It was therefore a perfectly understandable reaction that upon being confronted by a mysterious woman in black (who in the manner of mysterious black-clad women everywhere was certain to make his life far more complicated and perilous than it currently was), he tried to shoot her.
Conventional wisdom also argues against attempting to shoot prospective employers who aren't confirmed Multi-Level-Marketing representatives, but fortunately for both Number Five and our mystery woman (who it turns out was called The Handler because absolutely no one in this story is allowed a reasonable name) his penchant for shooting at people he'd only just met was precisely the reason she'd come in search of him.
On the subject of The Handler (AKA, **REDACTED**), The Hitchhiker's Guide to the Galaxy says: "Regional director of Temps Aeternalis corrections division. Head bitch in charge. If you see her you're about to die or be recruited, either way you're screwed." A small footnote link then takes the reader to the Guide's main entry for the Temps Aeternalis, (AKA "Temps Commission" AKA "The Real Reason Your Uncle Randolph Got That Safe Dropped On His Head") and from there goes on to describe the Aeternalis as an organization of self-appointed 'protectors' of the infinite number of timelines found across the universe, of which they believe it their job to monitor and, when necessary, take steps of a generally homicidal nature to preserve. The Guide goes on to declare this all a lot of bollocks, as the universe has it's own methods of temporal correction which it has been employing for a mindbogglingly unfathomably long time and had the situation well in hand long before a group of ape-like organisms with delusions of grandeur decided to stick their decidedly ape-like feet into things.
Regardless, it soon came to pass that the universe's foremost instrument of chaos was employed by the very organization whose raison d'être was attempting to eradicate chaos throughout every known point in space and time; a career at which he was almost suspiciously adept.
At this juncture it seems only fair to point out that on the subject of chaos the Hitchhiker's Guide to the Galaxy says: "An unpredictable cosmic prankster comprised of formless matter that existed before the creation of the universe. Loves a good joke. Particularly fond of butterflies and meringue." This last part is no great surprise to anyone who has actually attempted to make a consistently good meringue twice in a row.)
In any case, we shall now return to the other pivotal character in our story, D'Olo0rz, who was not at all happy to have spent the last forty years drunk, stranded, and carefully manipulating it's vessels thoughts only to have them get distracted in the home stretch by a bit of temporal murder. As such it took the first opportunity to remind Number Five that while traveling in time shooting people for fun and profit was all well and good, they had slightly more pressing concerns of the world-ending variety to attend to.
Unfortunately it was perhaps a side effect of spending nearly forty years drunker than the lemon at the bottom of a Pan Galactic Gargle Blaster that the calculations for the time dilation were off by about the same amount, and thus when Number Five finally completed the universal equation for time travel and was able to open a wormhole to the past his consciousness was unceremoniously transposed into the body of his thirteen year old self which, rather appropriately, traditionally marks the beginning of the most chaotic years of a human's life.
On the subject of being thirteen, The Hitchhiker's Guide to the Galaxy says: "Ugh." Both concise and eloquent, it is often quoted as the definitive word on the matter.
And so it was that Number Five, highly successful instrument of chaos, full-time lunatic, daytime drunk and the twelfth deadliest assassin in the universe (among carbon-based, bipedal lifeforms possessing three arms or less) was no longer tall enough to reach the top of the refrigerator, and had a bedtime of 10pm on weekdays.