diff -u b/core/modules/help_topics/help_topics.api.php b/core/modules/help_topics/help_topics.api.php
--- b/core/modules/help_topics/help_topics.api.php
+++ b/core/modules/help_topics/help_topics.api.php
@@ -14,10 +14,11 @@
  * Perform alterations on help topic definitions.
  *
  * @param array $info
- *   Array of information exposed by help topic plugins.
+ *   Array of help topic plugin definitions keyed by their plugin ID.
  */
-function hook_help_topics_info_alter(&$info) {
-  // @todo
+function hook_help_topics_info_alter(array &$info) {
+  // Alter the help topic to be displayed on admin/help.
+  $info['example.help_topic']['top_level'] = TRUE;
 }
 
 /**
diff -u b/core/modules/help_topics/src/Controller/HelpTopicPluginController.php b/core/modules/help_topics/src/Controller/HelpTopicPluginController.php
--- b/core/modules/help_topics/src/Controller/HelpTopicPluginController.php
+++ b/core/modules/help_topics/src/Controller/HelpTopicPluginController.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\help_topics\Controller;
 
+use Drupal\Component\Utility\SortArray;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Url;
 use Drupal\help_topics\HelpTopicPluginManagerInterface;
@@ -86,9 +87,6 @@
     $related = $help_topic->getRelated();
     foreach ($related as $other_id) {
       if ($other_id !== $id) {
-        if (!$this->helpTopicPluginManager->hasDefinition($other_id)) {
-          break;
-        }
         /** @var \Drupal\help_topics\HelpTopicPluginInterface $topic */
         $topic = $this->helpTopicPluginManager->createInstance($other_id);
         $links[$other_id] = [
@@ -100,12 +98,7 @@
     }
 
     if (count($links)) {
-      uasort($links, function ($a, $b) {
-        if ($a['title'] == $b['title']) {
-          return 0;
-        }
-        return ($a['title'] < $b['title']) ? -1 : 1;
-      });
+      uasort($links, [SortArray::class, 'sortByTitleElement']);
       $build['#related'] = [
         '#theme' => 'links__related',
         '#heading' => [
diff -u b/core/modules/help_topics/src/HelpTopicDiscovery.php b/core/modules/help_topics/src/HelpTopicDiscovery.php
--- b/core/modules/help_topics/src/HelpTopicDiscovery.php
+++ b/core/modules/help_topics/src/HelpTopicDiscovery.php
@@ -98,8 +98,9 @@
         // The plugin ID begins with provider.
         list($file_name_provider,) = explode('.', $plugin_id, 2);
         // Only the Help Topics module can provider help for other extensions.
-        // @todo Remove help_topics special case once Help Topics is stable and
-        //   core modules can provide their own help topics.
+        // @todo https://www.drupal.org/project/drupal/issues/3025577 Remove
+        //   help_topics special case once Help Topics is stable and core
+        //   modules can provide their own help topics.
         if ($provider !== 'help_topics' && $provider !== $file_name_provider) {
           throw new DiscoveryException("The $file should begin with '$provider.'");
         }
@@ -131,7 +132,7 @@
           }
         }
         if (!isset($data['label'])) {
-          throw new DiscoveryException("The $file contains does not contain the required meta tag with name='label'");
+          throw new DiscoveryException("The $file contains does not contain the required meta tag with name='help_topic:label'");
         }
 
         $all[$provider][$data['id']] = $data;
diff -u b/core/modules/help_topics/src/HelpTopicPluginManager.php b/core/modules/help_topics/src/HelpTopicPluginManager.php
--- b/core/modules/help_topics/src/HelpTopicPluginManager.php
+++ b/core/modules/help_topics/src/HelpTopicPluginManager.php
@@ -92,9 +92,13 @@
    */
   protected function getDiscovery() {
     if (!isset($this->discovery)) {
-      // We want to find help topic plugins in both modules and themes in
+      // We want to find help topic plugins in core, modules and themes in
       // a sub-directory called help_topics.
-      $directories = array_merge($this->moduleHandler->getModuleDirectories(), $this->themeHandler->getThemeDirectories());
+      $directories = array_merge(
+        ['core'],
+        $this->moduleHandler->getModuleDirectories(),
+        $this->themeHandler->getThemeDirectories()
+      );
 
       $directories = array_map(function ($dir) {
         return [$dir . '/help_topics'];
diff -u b/core/modules/help_topics/src/Plugin/HelpSection/HelpTopicSection.php b/core/modules/help_topics/src/Plugin/HelpSection/HelpTopicSection.php
--- b/core/modules/help_topics/src/Plugin/HelpSection/HelpTopicSection.php
+++ b/core/modules/help_topics/src/Plugin/HelpSection/HelpTopicSection.php
@@ -122,14 +122,15 @@
         }
       }
 
-      // Sort the top level topics by label and if they match by plugin ID.
+      // Sort the top level topics by label and, if the labels match, then by
+      // plugin ID.
       usort($this->topLevelPlugins, function (HelpTopicPluginInterface $a, HelpTopicPluginInterface $b) {
         $a_label = (string) $a->getLabel();
         $b_label = (string) $b->getLabel();
         if ($a_label === $b_label) {
           return $a->getPluginId() < $b->getPluginId() ? -1 : 1;
         }
-        return $a_label < $b_label ? -1 : 1;
+        return strnatcasecmp($a_label, $b_label);
       });
     }
     return $this->topLevelPlugins;
diff -u b/core/modules/help_topics/tests/modules/help_topics_test/help_topics_test.module b/core/modules/help_topics/tests/modules/help_topics_test/help_topics_test.module
--- b/core/modules/help_topics/tests/modules/help_topics_test/help_topics_test.module
+++ b/core/modules/help_topics/tests/modules/help_topics_test/help_topics_test.module
@@ -18,0 +19,7 @@
+
+/**
+ * Implements hook_help_topics_info_alter().
+ */
+function help_topics_test_help_topics_info_alter(array &$info) {
+  $info['help_topics_test.test']['top_level'] = \Drupal::state()->get('help_topics_test.test:top_level', TRUE);
+}
diff -u b/core/modules/help_topics/tests/src/Functional/HelpTopicTest.php b/core/modules/help_topics/tests/src/Functional/HelpTopicTest.php
--- b/core/modules/help_topics/tests/src/Functional/HelpTopicTest.php
+++ b/core/modules/help_topics/tests/src/Functional/HelpTopicTest.php
@@ -102,6 +102,16 @@
       $pos = $new_pos;
     }
 
+    // Ensure the plugin manager alter hook works as expected.
+    $session->linkExists('ABC Help Test module');
+    \Drupal::state()->set('help_topics_test.test:top_level', FALSE);
+    \Drupal::service('plugin.manager.help_topic')->clearCachedDefinitions();
+    $this->drupalGet('admin/help');
+    $session->linkNotExists('ABC Help Test module');
+    \Drupal::state()->set('help_topics_test.test:top_level', TRUE);
+    \Drupal::service('plugin.manager.help_topic')->clearCachedDefinitions();
+    $this->drupalGet('admin/help');
+
     // Ensure all the expected links are present before uninstalling.
     $session->linkExists('ABC Help Test module');
     $session->linkExists('ABC Help Test');
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/help_topics/tests/src/Unit/HelpTopicDiscoveryTest.php
@@ -0,0 +1,157 @@
+<?php
+
+namespace Drupal\Tests\filter\Unit;
+
+use Drupal\Component\Discovery\DiscoveryException;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\help_topics\HelpTopicDiscovery;
+use Drupal\help_topics\HelpTopicTwig;
+use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
+
+/**
+ * @coversDefaultClass \Drupal\help_topics\HelpTopicDiscovery
+ * @group help_topics
+ */
+class HelpTopicDiscoveryTest extends UnitTestCase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+  }
+
+  /**
+   * @covers ::findAll
+   */
+  public function testDiscoveryExceptionProviderMismatch() {
+    vfsStream::setup('root');
+    vfsStream::create([
+      'modules' => [
+        'foo' => [
+          'help_topics' => [
+            // The content of the help topic does not matter.
+            'test.topic.html.twig' => '',
+          ],
+        ],
+      ],
+    ]);
+    $discovery = new HelpTopicDiscovery(['foo' => vfsStream::url('root/modules/foo/help_topics')]);
+
+    $this->setExpectedException(DiscoveryException::class, "The vfs://root/modules/foo/help_topics/test.topic.html.twig should begin with 'foo.'");
+    $discovery->getDefinitions();
+  }
+
+  /**
+ * @covers ::findAll
+ */
+  public function testDiscoveryExceptionMissingLabelMetaTag() {
+    vfsStream::setup('root');
+
+    vfsStream::create([
+      'modules' => [
+        'test' => [
+          'help_topics' => [
+            // The content of the help topic does not matter.
+            'test.topic.html.twig' => '',
+          ],
+        ],
+      ],
+    ]);
+    $discovery = new HelpTopicDiscovery(['test' => vfsStream::url('root/modules/test/help_topics')]);
+
+    $this->setExpectedException(DiscoveryException::class, "The vfs://root/modules/test/help_topics/test.topic.html.twig contains does not contain the required meta tag with name='help_topic:label'");
+    $discovery->getDefinitions();
+  }
+
+  /**
+   * @covers ::findAll
+   */
+  public function testDiscoveryExceptionInvalidMetaTag() {
+    vfsStream::setup('root');
+    // Note a blank line is required after the last meta tag otherwise the last
+    // meta tag is not parsed.
+    $topic_content = <<<'EOS'
+<meta name="help_topic:label" content="A label"/>
+<meta name="help_topic:foo" content="bar"/>
+
+EOS;
+
+    vfsStream::create([
+      'modules' => [
+        'test' => [
+          'help_topics' => [
+            'test.topic.html.twig' => $topic_content,
+          ],
+        ],
+      ],
+    ]);
+    $discovery = new HelpTopicDiscovery(['test' => vfsStream::url('root/modules/test/help_topics')]);
+
+    $this->setExpectedException(DiscoveryException::class, "The vfs://root/modules/test/help_topics/test.topic.html.twig contains invalid meta tag with name='foo'");
+    $discovery->getDefinitions();
+  }
+
+  /**
+ * @covers ::findAll
+ */
+  public function testHelpTopicsExtensionProviderSpecialCase() {
+    vfsStream::setup('root');
+    $topic_content = <<<'EOS'
+<meta name="help_topic:label" content="Test"/>
+<h2>Test</h2>
+EOS;
+
+    vfsStream::create([
+      'modules' => [
+        'help_topics' => [
+          'help_topics' => [
+            'core.topic.html.twig' => $topic_content,
+          ],
+        ],
+      ],
+    ]);
+    $discovery = new HelpTopicDiscovery(['help_topics' => vfsStream::url('root/modules/help_topics/help_topics')]);
+    $this->assertArrayHasKey('core.topic', $discovery->getDefinitions());
+  }
+
+  /**
+   * @covers ::findAll
+   */
+  public function testHelpTopicsDefinition() {
+    $container = new ContainerBuilder();
+    $container->set('string_translation', $this->getStringTranslationStub());
+    \Drupal::setContainer($container);
+
+    vfsStream::setup('root');
+    $topic_content = <<<'EOS'
+<meta name="help_topic:label" content="Test"/>
+<meta name="help_topic:top_level"/>
+<meta name="help_topic:related" content="one,two,three"/>
+<h2>Test</h2>
+EOS;
+
+    vfsStream::create([
+      'modules' => [
+        'foo' => [
+          'help_topics' => [
+            'foo.topic.html.twig' => $topic_content,
+          ],
+        ],
+      ],
+    ]);
+    $discovery = new HelpTopicDiscovery(['foo' => vfsStream::url('root/modules/foo/help_topics')]);
+    $definition = $discovery->getDefinitions()['foo.topic'];
+    $this->assertEquals('Test', $definition['label']);
+    $this->assertInstanceOf(TranslatableMarkup::class, $definition['label']);
+    $this->assertSame(TRUE, $definition['top_level']);
+    $this->assertSame(['one', 'two', 'three'], $definition['related']);
+    $this->assertSame('foo', $definition['provider']);
+    $this->assertSame(HelpTopicTwig::class, $definition['class']);
+    $this->assertSame(vfsStream::url('root/modules/foo/help_topics/foo.topic.html.twig'), $definition['_discovered_file_path']);
+    $this->assertSame('foo.topic', $definition['id']);
+  }
+
+}