So Long, and Thanks for All the Fish is the fourth installment in the increasingly inaccurately-namedThe Hitchhiker's Guide to the Galaxy trilogy; it's set eight years since the beginning of the first book, but the amount of time that has passed between the end of the third, Life, the Universe and Everything, and the beginning of this one is not made clear. The plot concerns series protagonist Arthur Dent, who is very surprised to discover that his homeworld, planet Earth, seems to have suddenly sprung back into existence even though the alien Vogons destroyed it eight years ago to make way for an interstellar bypass.He explores his old Islington home and, seeing everything back where it should be, starts to suspect that his memories of space travel may have been nothing more than hallucinations; after all, the people say, a nearby water source was found to be accidentally contaminated by hallucinogens. Arthur returns to his old ways and even strikes up a meaningful romance with Fenchurch, a beautiful young woman who seems just right for him. However, something is not quite right. Every dolphin on Earth seems to have mysteriously disappeared, as has his old friend Ford Prefect, and the appearance of a strange fishbowl on his doorstep sends Arthur, and Fenchurch, on a quest to discover the truth...Notable for featuring God's Final Message to His Creation.Preceded by Life, the Universe and Everything. Succeeded by Mostly Harmless.
So Long, and Thanks for All the Fish provides examples of the following tropes:
Bavarian Fire Drill: Ford pulls this to get himself, Arthur and Fenchurch onto a spaceship and off Earth.
Best Before Decade: Arthur returns to his home after years in space, and eats "the three least green furry things" in the fridge for breakfast.
Brick Joke: This story starts with an almost identical narration to the first novel, before revealing in the first few chapters that Fenchurch was the woman mentioned in it.
Creator Breakdown: Adams devotes one entire chapter to how fed up he is with people asking him about Arthur Dent's sex life. With a bit of a dig at people who kept saying, while he was writing this book, "You should put a Zaphod bit in here", by sarcastically concluding that those who aren't interested in Arthur's sex life should skip to the end, which has Marvin in it. Though an "entire chapter" doesn't mean all that much, since there are quite a few one-page chapters in the series. There is even a chapter in this book that consists of an entire three sentences.
Democracy Is Bad: "On its world, the people are people. The leaders are lizards. The people hate the lizards and the lizards rule the people, if they didn't vote for a lizard, the wrong lizard might get in." Though one could argue this is more of an argument against first-past-the-post voting than against democracy in general.
First Contact: Ford arrives on Earth in a ship that creates this scenario. He then sneaks himself, Arthur and Fenchurch on board when it's leaving.
Flight of Romance: Arthur and Fenchurch take this up to extremes, not just falling in love, but also, um, really getting to know each other.
If I Had a Nickel: Ford says, "If I had a pound for every time one part of the universe looked at another part of the universe and said, 'That's terrible,' I wouldn't be sitting here like a lemon asking you to bring me a cup of coffee, but I don't and I am."
If you took a couple of David Bowies and stuck one of the David Bowies on the top of the other David Bowie, then attached another David Bowie to the end of each of the arms of the upper of the first two David Bowies and wrapped the whole business up in a dirty beach robe you would then have something which didn't exactly look like John Watson, but which those who knew him would find hauntingly familiar.
Timey Wimey Ball: This book is set a subjective eight years for Arthur after he first left the Earth at the beginning of the first book. As he spent five of those years on prehistoric Earth, it can be inferred that this is about three years after in "real time". But on Earth itself, just under six months appear to have passed (from early September to mid-February). Then again, when he left Earth, it was destroyed. It could be inferred that it took the dolphins two and a half years to get it set up so Earth could continue without anyone noticing the difference.
But his smile when he turned it on you was quite remarkable. It seemed to be composed of all the worst things that life can do to you, but which, when he briefly reassembled them in that particular order on his face, made you suddenly fee, "Oh. Well that's all right then."