AutoCollapseWikipediaPlotSection.user.js

// ==UserScript==
// @name         Auto Collapse Wikipedia "Plot" Section
// @namespace    https://greasyfork.org/en/users/85671-jcunews
// @version      1.0.1
// @license      AGPLv3
// @author       jcunews
// @description  https://www.reddit.com/r/userscripts/comments/n72ppa/request_collapse_the_plot_paragraph_by_default_on/
// @match        https://*.wikipedia.org/wiki/*
// @grant        none
// ==/UserScript==

((a, b, c, d, l) => {
  if (a = document.querySelector("#Plot.mw-headline")) {
    b = a.nextElementSibling ? a : a.parentNode;
    b.insertAdjacentHTML("beforeend", '<a href="javascript:void(0)" style="margin-left:2em;font-size:10pt;font-weight:normal">[Expand]</a>');
    b.lastChild.onclick = function() {
      l.forEach(e => {
        e.style.display = e.dataset.display;
      });
      this.remove();
    };
    d = (c = Array.from(document.querySelectorAll(".mw-headline,.navbox"))).indexOf(a);
    if (d >= 0) {
      if (!(c = c[d + 1]).nextElementSibling) c = c.parentNode;
    } else c = null;
    l = [];
    while ((b = b.nextElementSibling) && (b !== c)) {
      l.push(b);
      b.dataset.display = b.style.display;
      b.style.display = "none";
    }
  }
})();