Though the exact story of the album is very ambiguous and open to interpretation (the following summary of the album's story being one of the many existing interpretations), the general concept of The Downward Spiral is that it chronicles a nameless protagonist's descent into an existential crisis which descends further into madness, his attempts to find solace in control (sexual or otherwise), and his disillusionment with and (symbolic, literal, or delusional) destruction of those to whom he was once close, God, society ... and ultimately himself.
The first track, "Mr. Self Destruct", is pretty much a synopsis of the protagonist's self-destructive personality. Then the spiral starts with "Piggy", when he loses his girlfriend (rather that she cheated on him than she died), who was pretty much his only tether to reality, and then goes into a completely freaking insane festival of ear rape and horror. It goes on to talk (or rather scream) about his God Is Dead philosophy and his hate for religion in "Heresy", then establishes the fact that he's a chronic sociopath in "March of the Pigs".
After that it flashes back to when the relationship with his girlfriend grew into despair and how he was utterly dependent on her and the trouble that she gave him in "Closer" (this track gathered a serious Misaimed Fandom) and how he would give up everything just to abuse her insides. Then it returns to the current happenings, as "Ruiner" starts playing, which pretty much goes through his interactions with a virus-infected person known as the "ruiner", who he thinks has taken his place, but he suddenly realizes that he's becoming just what he hates as the utter fucking insanity and screams of the damned start stabbing away at what's left of your sanity by this track. And then he kills the ruiner.
It then goes from that to "The Becoming" and "I Do Not Want This", the latter which is seemingly much less terrifying, as he further states the psycho-social battle he's fighting, that would be the battle against someone known as "the Machine", which would be a colder version of him that didn't have emotions and obviously, that scares the main character to death. This track is when you start to be able to hear the album decaying and rotting away as he gets closer to the bottom of the spiral. He fights and argues with the Machine, and eventually he decides he wants to "Know everything, be everywhere and fuck everyone in the world" and wants to "Do something that matters".
And then it goes into "Big Man with a Gun", which could be perceived as a rape scenario, or otherwise the protagonist going on a major power-trip, or actually him going insane and violent. The song abruptly ends and goes into "A Warm Place", which is both a sudden realization that what he just did both didn't work and was kind of a dick move and the calm before the storm (the storm being the bottom of the spiral). Some people also se "Eraser" states that he wants to end his life — but he isn't ready to do the deed — and the great guilt he feels for all of the crap he's done in the past. The track abruptly ends and then goes into "Reptile", as he hooks up with a girl that he's not too fond of like his ex girlfriend, so it might be another flashback (this or maybe he just got himself a hooker) for the sex — and how he realizes this is rock-bottom for him.
The next track is the Title Track. This can be taken more than one way. Either you can go for the Exactly What It Says on the Tin ending and he killed himself as a voice, presumably the Machine, taunts him and narrates the event or you could say that this was simply a suicide attempt and he didn't succeed in killing himself and is still alive (and that's how he's talking to you during "Hurt"). The decay and rotting is at its height in this track, with the barely recognizable drums and guitar pounding away and the muffled final retaliation of our main character screaming "NO" and the highly distorted voice of the Machine taunting him as he attempts to end his life.
Then it goes into the final track, "Hurt". For about twenty seconds, there's this noise that sounds like some leftover white noise from the title track that quietly hums in the background. Then the lyrics start along with the instruments. Unlike most of the album, this track doesn't seem to take place directly after the track before it. It seems like it's at least a month or two later, stating the final ballad of our main character as he tries to hold on to what's left of his humanity and silently retaliate against the Machine, although he seems to have almost completely given up by this point. The album closes with him stating "If I could start again a million miles away, I would keep myself" and then it goes into a highly distorted and disconcerting guitar riff and he continues the sentence to state "I would find a way." For another minute or two, it just hold the metallic screeching of that guitar, possibly to signify that the Machine has taken over and all that's left is the cold metal, but then suddenly stops and plays the loop that played for the first 20 seconds of the track, possibly to say that the humanity is still in there somewhere.
