The story, of course, begins with a falsehood.
Take in as much air as you can.
This story should last about as long as you can hold your breath, and then just a little bit longer.
This story clocks in at somewhere in the 3000 word range, meaning that even at 2500 words, and read at the cheetah-like 600 words per minute, it would still take five minutes to get through the story. I'm not sure what the lung capacity for the average person is, but I imagine five minutes is pushing it. It's even worse if the story is out loud. Even John Moschitta, Jr. (who most in my generation will recognize as The Micro Machine Man) only clocks in at 586 words per minute, or slightly over five minutes. It's arguable that the narrator himself, who describes his lung capacity as "huge," may manage to hold his breath for that long, but it's not advisable for the rest of us.
"Guts" is essentially three stories about masturbation, each more gruesome and twisted than the one before it. The first is a kid of thirteen years old who was always "jonesing for a better way to get his rocks off." The kid hears about stimulating the prostate gland to induce orgasm and attempts it using a carrot and some Vaseline. When he gets called to dinner, he hides the carrot but comes back to see that it has disappeared along with is dirty laundry. From that day onward, a sense of shame and foreboding hangs over his relationship with is parents.
The second kid hears from his older brother that orgasm can be intesified through the insertion of a thin smooth tube into the urethra. One evening, while high he uses a thin tube of wax, which he's taken from a candle dripping, in order to try out this theory. Just as he's about to climax, he realizes that the tube has disappeared. A few hours later, his abdomen begins to hurt. After being taken to the hospital, an X-ray reveals that the tube has fallen into his bladder and is absorbing minerals, growing rough and damaging the inside of his bladder. His parents have to dip into his college fund to pay for the operation. "One stupid mistake, and how he'll never be a lawyer," the narrator tells us.
The last story is the narrator's own. He used to masturbate in the pool while sitting naked on the drain at the very bottom, which he called Pearl Diving due to his practice of snatching all the floating ejaculate from the water afterwards. As he finished one day and goes to kick off for air, he finds that he's stuck. Looking back, he sees "some kind of snake, blue-white and braided with veins" has come up from the pool drain and is latched on to his butt.
This is where the novel achieves something almost Poe-like. As the narrator realizes that the snake is actually his small intestine--pulled out by the suction of the drain--and begins to consider his actions and the consequences, the story achieves a degree of claustrophobia, compounded by the gruesome body horror, that makes it difficult to forget or dismiss. The kid eventually does what he has to in order to survive, telling us in a line that is perhaps a little too clever: "If I told you what it tasted like, you would never, ever again eat calamari." (Of course, "If I told you what it tasted like, you would never, ever again eat chitterlings" would be more accurate, but probably leave many confused.)
Afterwards, his family lives in denial, his dad attributing the mess to a dog that fell into the pool. It is just at the end that Palahniuk/the narrator seems to overreach, laying out the final horror wherein his sister misses her period, implying that she has become pregnant from the ejaculate in the pool. It is at this point that the story crosses over from extreme but plausible to simply ridiculous, for reasons I won't get into at this point. However, overall it's a brilliantly gruesome story, that manages to inject dark humor and a certain melancholy into its shocking proceedings.