diff --git a/get_data.lib.php b/get_data.lib.php
index 8d631b4..62c8a4f 100644
--- a/get_data.lib.php
+++ b/get_data.lib.php
@@ -3,6 +3,13 @@ require_once('open_db.lib.php');
 require_once(__DIR__ . '/lib/purify.inc.php');
 require_once(__DIR__ . '/html.lib.php');
 
+// The longest story HTML, in bytes. HTML Purifier needs seconds for a few megabytes.
+define('MAX_STORY_BYTES', 65536);
+
+function story_too_long($story) {
+  return strlen((string)$story) > MAX_STORY_BYTES;
+}
+
 function display_stories($id = NULL) {
   global $db, $is_admin, $editable, $action;
   if (is_null($id))
@@ -10,17 +17,20 @@ function display_stories($id = NULL) {
   $search = get_search_args();
   $page   = '';
   $nolink = NULL;
+  $posted = FALSE;
 
   if ($action == 'preview') {
     global $file_src;
     $date = 'Awaiting posting';
     $result = get_post_data(TRUE);
     $nolink = TRUE;
+    $posted = TRUE;
   } else if ($action == 'submit') {
     $date = 'Posted '.gmdate("Y-m-d H:i");
     if (!$is_admin) {
       $result = get_post_data(TRUE);
       $nolink = TRUE;
+      $posted = TRUE;
     } else {
       $result = get_query($id);
     }
@@ -90,6 +100,8 @@ date("Y-m-d H:i", strtotime($entry['date'])) . '</time>';
       } else {
         $page .= '<img src="/display?id='.$entry_id.'" alt="'.$title.'" />';
       }
+    } else if ($posted && story_too_long($entry['story'])) {
+      $page .= '<p>The story is too long to show.</p>'."\n";
     } else {
       $page .= purify_story($entry['story'])."\n";
     }
diff --git a/publish/post.lib.php b/publish/post.lib.php
index 4d9b2c9..90aef93 100644
--- a/publish/post.lib.php
+++ b/publish/post.lib.php
@@ -206,6 +206,10 @@ function validate_data() {
     $validate['title'] = "You have not submitted a title.";
   }
 
+  if (story_too_long($story)) {
+    $validate['story'] = 'The story is too long. Please shorten it.';
+  }
+
   if ($remove_img && !$story) {
     $validate['story'] = "You've tried to remove an image without replacing it with text.";
   }
diff --git a/tests/php/Unit/DisplayStoriesTest.php b/tests/php/Unit/DisplayStoriesTest.php
index a7c7f62..bfe0f7d 100644
--- a/tests/php/Unit/DisplayStoriesTest.php
+++ b/tests/php/Unit/DisplayStoriesTest.php
@@ -111,6 +111,15 @@ class DisplayStoriesTest extends TestCase {
     $this->assertStringNotContainsString('x()', $page);
   }
 
+  // HTML Purifier needs seconds for a few megabytes, so a posted story over the limit is not shown
+  public function testPreviewDoesNotShowATooLongStory(): void {
+    $GLOBALS['action'] = 'preview';
+    $this->request(array(), array('title' => 'Long', 'author' => 'Me', 'story' => '<p><b>x</b></p>' . str_repeat('a', 65536)));
+    $page = display_stories();
+    $this->assertStringContainsString('<p>The story is too long to show.</p>', $page);
+    $this->assertStringNotContainsString('<b>x</b>', $page);
+  }
+
   public function testPreviewShowsAPendingImage(): void {
     $GLOBALS['action'] = 'preview';
     $GLOBALS['file_src'] = '/display?pending="><script>';
diff --git a/tests/php/Unit/ValidateDataTest.php b/tests/php/Unit/ValidateDataTest.php
index 7c1315e..bc9ecb3 100644
--- a/tests/php/Unit/ValidateDataTest.php
+++ b/tests/php/Unit/ValidateDataTest.php
@@ -35,6 +35,12 @@ class ValidateDataTest extends TestCase {
     $this->assertSame(array(), $this->validate(array('remove_img' => '1')));
   }
 
+  public function testATooLongStoryIsRefused(): void {
+    $this->assertSame(array(), $this->validate(array('story' => str_repeat('a', 65536))));
+    $this->assertSame(array('story' => 'The story is too long. Please shorten it.'),
+                      $this->validate(array('story' => str_repeat('a', 65537))));
+  }
+
   public function testVisitorsNeedTheCaptcha(): void {
     FakeRecaptcha::install('{"tokenProperties":{"valid":false}}');
     $this->assertSame(array('captcha' => 'Please complete the captcha.'), $this->validate(array('g-recaptcha-response' => 'r'), FALSE));