diff --git a/lib/rdoc/generator/darkfish.rb b/lib/rdoc/generator/darkfish.rb
index 08f2b85e3b..cfd46b5ed4 100644
--- a/lib/rdoc/generator/darkfish.rb
+++ b/lib/rdoc/generator/darkfish.rb
@@ -316,11 +316,14 @@ def generate_index
     asset_rel_prefix = rel_prefix + @asset_rel_path
 
     @title = @options.title
+    @main_page = @files.find { |f| f.full_name == @options.main_page }
 
     render_template template_file, out_file do |io|
       here = binding
       # suppress 1.9.3 warning
       here.local_variable_set(:asset_rel_prefix, asset_rel_prefix)
+      # some partials rely on the presence of current variable to render
+      here.local_variable_set(:current, @main_page) if @main_page
       here
     end
   rescue => e
diff --git a/lib/rdoc/generator/template/darkfish/index.rhtml b/lib/rdoc/generator/template/darkfish/index.rhtml
index a5c0dd54da..beaab9570f 100644
--- a/lib/rdoc/generator/template/darkfish/index.rhtml
+++ b/lib/rdoc/generator/template/darkfish/index.rhtml
@@ -7,6 +7,7 @@
     <%= render '_sidebar_search.rhtml' %>
   </div>
 
+  <%= render '_sidebar_table_of_contents.rhtml' if defined?(current) %>
   <%= render '_sidebar_pages.rhtml' %>
   <%= render '_sidebar_classes.rhtml' %>
 
@@ -14,10 +15,9 @@
 </nav>
 
 <main role="main">
-<%- if @options.main_page and
-      main_page = @files.find { |f| f.full_name == @options.main_page } then %>
-<%= main_page.description %>
+<%- if @main_page %>
+  <%= @main_page.description %>
 <%- else -%>
-<p>This is the API documentation for <%= h @title %>.
+  <p>This is the API documentation for <%= h @title %>.
 <%- end -%>
 </main>
diff --git a/test/rdoc/test_rdoc_generator_darkfish.rb b/test/rdoc/test_rdoc_generator_darkfish.rb
index 0a9be7e4ee..cedc876b4e 100644
--- a/test/rdoc/test_rdoc_generator_darkfish.rb
+++ b/test/rdoc/test_rdoc_generator_darkfish.rb
@@ -134,6 +134,88 @@ def test_generate
     )
   end
 
+  def test_generate_index_with_main_page
+    top_level = @store.add_file 'file.rb'
+    top_level.comment = <<~RDOC
+    = Heading 1
+    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
+    == Heading 1.1
+    tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
+    === Heading 1.1.1
+    quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+    ==== Heading 1.1.1.1
+    consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+    == Heading 1.2
+    cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat
+    == Heading 1.3
+    non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+    === Heading 1.3.1
+    etc etc...
+    RDOC
+
+    @options.main_page = 'file.rb'
+    @options.title = 'My awesome Ruby project'
+
+    @g.generate
+
+    assert_file 'index.html'
+    assert_file 'table_of_contents.html'
+    assert_file 'js/search_index.js'
+
+    assert_hard_link 'css/rdoc.css'
+    assert_hard_link 'css/fonts.css'
+
+    assert_hard_link 'fonts/SourceCodePro-Bold.ttf'
+    assert_hard_link 'fonts/SourceCodePro-Regular.ttf'
+
+    index_html = File.binread('index.html')
+
+    assert_include index_html, "<h3>Table of Contents</h3>"
+    assert_include index_html, '<h1 id="label-Heading+1">Heading 1'
+    # When there's a main page, the default description should not be shown
+    assert_not_include index_html, 'This is the API documentation for My awesome Ruby project.'
+  end
+
+  def test_generate_index_without_main_page
+    top_level = @store.add_file 'file.rb'
+    top_level.comment = <<~RDOC
+    = Heading 1
+    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
+    == Heading 1.1
+    tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
+    === Heading 1.1.1
+    quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+    ==== Heading 1.1.1.1
+    consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+    == Heading 1.2
+    cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat
+    == Heading 1.3
+    non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+    === Heading 1.3.1
+    etc etc...
+    RDOC
+
+    @options.title = 'My awesome Ruby project'
+
+    @g.generate
+
+    assert_file 'index.html'
+    assert_file 'table_of_contents.html'
+    assert_file 'js/search_index.js'
+
+    assert_hard_link 'css/rdoc.css'
+    assert_hard_link 'css/fonts.css'
+
+    assert_hard_link 'fonts/SourceCodePro-Bold.ttf'
+    assert_hard_link 'fonts/SourceCodePro-Regular.ttf'
+
+    index_html = File.binread('index.html')
+
+    # If there is no main page, the index page should not have a table of contents
+    assert_not_include index_html, "<h3>Table of Contents</h3>"
+    assert_include index_html, 'This is the API documentation for My awesome Ruby project.'
+  end
+
   def test_generate_page
     @store.add_file 'outer.rdoc', parser: RDoc::Parser::Simple
     @store.add_file 'outer/inner.rdoc', parser: RDoc::Parser::Simple