diff --git a/mkdocs_literate_nav/plugin.py b/mkdocs_literate_nav/plugin.py
index 773b2a5..807ac48 100644
--- a/mkdocs_literate_nav/plugin.py
+++ b/mkdocs_literate_nav/plugin.py
@@ -13,6 +13,11 @@
 import mkdocs.structure.nav
 import mkdocs.structure.pages
 
+try:
+    from mkdocs.exceptions import PluginError
+except ImportError:
+    PluginError = SystemExit
+
 from mkdocs_literate_nav import parser
 
 log = logging.getLogger(f"mkdocs.plugins.{__name__}")
@@ -21,7 +26,7 @@
 
 class LiterateNavPlugin(mkdocs.plugins.BasePlugin):
     config_scheme = (
-        ("nav_file", mkdocs.config.config_options.Type(str, default=None)),
+        ("nav_file", mkdocs.config.config_options.Type(str, default="SUMMARY.md")),
         ("implicit_index", mkdocs.config.config_options.Type(bool, default=False)),
     )
 
@@ -60,7 +65,7 @@ def resolve_directories_in_nav(
     """
 
     def read_index_of_dir(path: str) -> Optional[str]:
-        file = find_index_of_dir(files, path, nav_file_name)
+        file = files.get_file_from_path(os.path.join(path, nav_file_name))
         if not file:
             return None
         log.debug(f"Navigation for {path!r} based on {file.src_path!r}.")
@@ -93,24 +98,6 @@ def try_resolve_directory(path: str):
     return convert_strings_in_nav(nav_data, try_resolve_directory)
 
 
-def find_index_of_dir(
-    files: mkdocs.structure.files.Files, path: str, nav_file_name: Optional[str] = None
-) -> mkdocs.structure.files.File:
-    """Find the directory's index (or nav if specified) Markdown file.
-
-    If `nav_file` is configured, unconditionally get that file from this dir (could be None).
-    Else try README.md. Else try whatever maps to the index (effectively index.md or readme.md).
-    """
-    if nav_file_name:
-        return files.get_file_from_path(os.path.join(path, nav_file_name))
-    f = files.get_file_from_path(os.path.join(path, "README.md"))
-    if f:
-        return f
-    for f in files.documentation_pages():
-        if os.path.split(f.src_path)[0] == path and f.name == "index":
-            return f
-
-
 def convert_strings_in_nav(nav_data, converter: Callable[[str], Optional[Any]]):
     """Walk a nav dict and replace strings in it with the callback."""
     if isinstance(nav_data, str):
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 5703857..439e382 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -21,7 +21,10 @@ def test_resolve_directories_in_nav(tmp_path_factory, use_directory_urls, golden
 
     with golden.capture_logs("mkdocs.plugins.mkdocs_literate_nav"):
         output = plugin.resolve_directories_in_nav(
-            golden.get("nav"), files, golden.get("nav_file_name"), golden.get("implicit_index")
+            golden.get("nav"),
+            files,
+            nav_file_name=golden.get("nav_file_name") or "index.md",
+            implicit_index=golden.get("implicit_index"),
         )
     assert output == golden.out["output"]
 
@@ -29,36 +32,3 @@ def test_resolve_directories_in_nav(tmp_path_factory, use_directory_urls, golden
 def _file_sort_key(f: File):
     parts = pathlib.PurePath(f.src_path).parts
     return tuple(chr(i != len(parts) - 1) + chr(f.name != "index") + p for i, p in enumerate(parts))
-
-
-@pytest.mark.parametrize("use_directory_urls", [True, False])
-def test_find_index_of_dir(tmp_path_factory, use_directory_urls):
-    src_dir, dest_dir = map(tmp_path_factory.mktemp, ["src", "dest"])
-    file_names = [
-        " README.md",
-        "README.md ",
-        "index.md",
-        "bad/index.html",
-        "bad/foo.md",
-        "bad/foo.index.md",
-        "both1/index.md",
-        "both1/README.md",
-        "both2/readme.md",
-        "both2/index.md",
-        "a/a/a/a/a/README.md",
-    ]
-    files = {f: File(f, src_dir, dest_dir, use_directory_urls) for f in file_names}
-    files_o = Files(list(files.values()))
-
-    assert plugin.find_index_of_dir(files_o, "") == files["index.md"]
-    assert plugin.find_index_of_dir(files_o, "nonexistent") is None
-    assert plugin.find_index_of_dir(files_o, "bad") is None
-    assert plugin.find_index_of_dir(files_o, "bad/foo") is None
-    assert plugin.find_index_of_dir(files_o, "bad/foo.md") is None
-    assert plugin.find_index_of_dir(files_o, "both1") == files["both1/README.md"]
-    assert plugin.find_index_of_dir(files_o, "both2") == files["both2/index.md"]
-    assert plugin.find_index_of_dir(files_o, "bad", "foo.md") == files["bad/foo.md"]
-    assert plugin.find_index_of_dir(files_o, "both2", "foo.md") is None
-    assert plugin.find_index_of_dir(files_o, "a/a/a/a", "a") is None
-    assert plugin.find_index_of_dir(files_o, "a/a/a/a/a") == files["a/a/a/a/a/README.md"]
-    assert plugin.find_index_of_dir(files_o, "a/a/a/a/a/a") is None