diff --git a/README.md b/README.md
index 945437c..fcc17f3 100644
--- a/README.md
+++ b/README.md
@@ -128,6 +128,11 @@ Invalid or non-numeric values fall back to 30 days.
 
 **Note:** _a value of 0 is **invalid**_.
 
+### Built-in diff
+This shows a _git-like_ diff of the current file version and any of that file backups.
+```
+mirro --diff file file.orig.20251121T163121
+```
 
 ## Installation
 
diff --git a/pyproject.toml b/pyproject.toml
index 7ebe5c2..44e461f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "mirro"
-version = "0.3.0"
+version = "0.4.0"
 description = "A safe editing wrapper: edits a temp copy, compares, and saves original backup if changed."
 authors = ["Marco D'Aleo <marco@marcodaleo.com>"]
 license = "GPL-3.0-or-later"
diff --git a/src/mirro/main.py b/src/mirro/main.py
index 29b28fa..7c7c2e5 100644
--- a/src/mirro/main.py
+++ b/src/mirro/main.py
@@ -4,6 +4,7 @@ import tempfile
 import subprocess
 import os
 import textwrap
+import difflib
 from pathlib import Path
 import time
 
@@ -49,6 +50,31 @@ def backup_original(
     return backup_path
 
 
+def strip_mirro_header(text: str) -> str:
+    """
+    Strip only mirro's backup header (if present).
+    Never removes shebangs or anything else.
+    """
+    lines = text.splitlines(keepends=True)
+
+    # If there's no mirro header, return the text unchanged
+    if not lines or not lines[0].startswith(
+        "# ---------------------------------------------------"
+    ):
+        return text
+
+    # Otherwise skip all header lines until the first blank line
+    i = 0
+    while i < len(lines):
+        if lines[i].strip() == "":
+            i += 1  # skip the blank separator line
+            break
+        i += 1
+
+    # 'i' now points to the first real line of the original file
+    return "".join(lines[i:])
+
+
 def main():
     parser = argparse.ArgumentParser(
         description="Safely edit a file with automatic original backup if changed."
@@ -87,9 +113,85 @@ def main():
         help="Prune backups older than MIRRO_BACKUPS_LIFE days, or 'all' to delete all backups",
     )
 
+    parser.add_argument(
+        "--diff",
+        nargs=2,
+        metavar=("FILE", "BACKUP"),
+        help="Show a unified diff between FILE and BACKUP and exit",
+    )
+
     # Parse only options. Leave everything else untouched.
     args, positional = parser.parse_known_args()
 
+    if args.diff:
+        file_arg, backup_arg = args.diff
+
+        file_path = Path(file_arg).expanduser().resolve()
+
+        # Resolve backup: if it’s not absolute or ~, treat it as a filename in the backup dir
+        if os.path.isabs(backup_arg) or backup_arg.startswith("~"):
+            backup_path = Path(backup_arg).expanduser().resolve()
+        else:
+            backup_dir = Path(args.backup_dir).expanduser().resolve()
+            backup_path = backup_dir / backup_arg
+
+        if not file_path.exists():
+            print(f"File not found: {file_path}")
+            return 1
+        if not backup_path.exists():
+            print(f"Backup not found: {backup_path}")
+            return 1
+
+        # Enforce same base filename while diffing
+        target_name = file_path.name
+        backup_name = backup_path.name
+
+        if not backup_name.startswith(target_name + ".orig."):
+            print(
+                f"Error: Backup '{backup_name}' does not match the file being diffed.\n"
+                f"Expected backup file starting with: {target_name}.orig."
+            )
+            return 1
+
+        original = file_path.read_text(
+            encoding="utf-8", errors="replace"
+        ).splitlines()
+        backup_raw = backup_path.read_text(encoding="utf-8", errors="replace")
+
+        backup_stripped = strip_mirro_header(backup_raw)
+        backup = backup_stripped.splitlines()
+
+        # Generate a clean diff (no trailing line noise)
+        diff = difflib.unified_diff(
+            backup,
+            original,
+            fromfile=f"a/{file_path.name}",
+            tofile=f"b/{file_path.name}",
+            lineterm="",
+        )
+
+        # Colors
+        RED = "\033[31m"
+        GREEN = "\033[32m"
+        CYAN = "\033[36m"
+        RESET = "\033[0m"
+
+        for line in diff:
+            if (
+                line.startswith("---")
+                or line.startswith("+++")
+                or line.startswith("@@")
+            ):
+                print(f"{CYAN}{line}{RESET}")
+            elif line.startswith("+"):
+                print(f"{GREEN}{line}{RESET}")
+            elif line.startswith("-"):
+                print(f"{RED}{line}{RESET}")
+            else:
+                print(line)
+
+        return
+
     if args.list:
         import pwd, grp
 
@@ -171,26 +273,7 @@ def main():
 
         # read and strip header
         raw = last.read_text(encoding="utf-8", errors="replace")
-        restored = []
-        skipping = True
-        for line in raw.splitlines(keepends=True):
-            # header ends at first blank line after the dashed line block
-            if skipping:
-                if line.strip() == "" and restored == []:
-                    # allow only after header
-                    continue
-                if line.startswith("#") or line.strip() == "":
-                    continue
-                skipping = False
-            restored.append(line)
-
-        # if header wasn't present, restored = raw
-        if not restored:
-            restored_text = raw
-        else:
-            restored_text = "".join(restored)
-
-        # write the restored file back
+        restored_text = strip_mirro_header(raw)
         target.write_text(restored_text, encoding="utf-8")
 
         print(f"Restored {target} from backup {last.name}")
diff --git a/tests/test_mirro.py b/tests/test_mirro.py
index fbb4753..f202bc7 100644
--- a/tests/test_mirro.py
+++ b/tests/test_mirro.py
@@ -48,6 +48,39 @@ def test_write_file(tmp_path):
     assert p.read_text(encoding="utf-8") == "data"
 
 
+# ============================================================
+# strip_mirro_header
+# ============================================================
+
+
+def test_strip_header_removes_header():
+    header_text = (
+        "# ---------------------------------------------------\n"
+        "# mirro backup\n"
+        "# something\n"
+        "# ---------------------------------------------------\n"
+        "\n"
+        "#!/bin/bash\n"
+        "echo hi\n"
+    )
+
+    out = mirro.strip_mirro_header(header_text)
+    assert out.startswith("#!/bin/bash")
+    assert "mirro backup" not in out
+
+
+def test_strip_header_preserves_shebang():
+    text = "#!/usr/bin/env python3\nprint('hi')\n"
+    out = mirro.strip_mirro_header(text)
+    assert out == text  # unchanged
+
+
+def test_strip_header_non_header_file():
+    text = "# just a comment\nvalue\n"
+    out = mirro.strip_mirro_header(text)
+    assert out == text
+
+
 # ============================================================
 # backup_original
 # ============================================================
@@ -352,11 +385,21 @@ def test_restore_last_success(tmp_path, capsys):
     d.mkdir()
     target = tmp_path / "t.txt"
 
+    mirro_header = (
+        "# ---------------------------------------------------\n"
+        "# mirro backup\n"
+        "# Original file: x\n"
+        "# Timestamp: test\n"
+        "# Delete this header if you want to restore the file\n"
+        "# ---------------------------------------------------\n"
+        "\n"
+    )
+
     b1 = d / "t.txt.orig.2020"
     b2 = d / "t.txt.orig.2021"
 
-    b1.write_text("# header\n\nold1")
-    b2.write_text("# header\n\nold2")
+    b1.write_text(mirro_header + "old1")
+    b2.write_text(mirro_header + "old2")
 
     # ensure newest
     os.utime(b2, (time.time(), time.time()))
@@ -392,7 +435,7 @@ def test_prune_all(tmp_path, capsys):
     assert not any(d.iterdir())
 
 
-def test_prune_numeric(tmp_path, capsys, monkeypatch):
+def test_prune_numeric(tmp_path, capsys):
     d = tmp_path / "bk"
     d.mkdir()
 
@@ -418,7 +461,7 @@ def test_prune_numeric(tmp_path, capsys, monkeypatch):
         mirro.main()
 
     out = capsys.readouterr().out
-    assert "Removed 1 backup" in out
+    assert "Removed 1" in out
     assert new.exists()
     assert not old.exists()
 
@@ -463,3 +506,62 @@ def test_prune_invalid_arg(tmp_path, capsys):
 
     assert result == 1
     assert "Invalid value for --prune-backups" in capsys.readouterr().out
+
+
+# ============================================================
+# --diff tests
+# ============================================================
+
+
+def test_diff_basic(tmp_path, capsys):
+    d = tmp_path / "bk"
+    d.mkdir()
+
+    file = tmp_path / "t.txt"
+    file.write_text("line1\nline2\n")
+
+    backup = d / "t.txt.orig.20250101T010203"
+    backup.write_text(
+        "# ---------------------------------------------------\n"
+        "# mirro backup\n"
+        "# whatever\n"
+        "\n"
+        "line1\nold\n"
+    )
+
+    with patch(
+        "sys.argv",
+        ["mirro", "--diff", str(file), backup.name, "--backup-dir", str(d)],
+    ):
+        mirro.main()
+
+    out = capsys.readouterr().out
+
+    assert "--- a/t.txt" in out
+    assert "+++ b/t.txt" in out
+    assert "@@" in out
+    assert "-old" in out
+    assert "+line2" in out
+
+
+def test_diff_wrong_backup_name_rejected(tmp_path, capsys):
+    d = tmp_path / "bk"
+    d.mkdir()
+
+    file = tmp_path / "foo.txt"
+    file.write_text("hello\n")
+
+    bad = d / "bar.txt.orig.20250101T010203"
+    bad.write_text("stuff\n")
+
+    with patch(
+        "sys.argv",
+        ["mirro", "--diff", str(file), bad.name, "--backup-dir", str(d)],
+    ):
+        result = mirro.main()
+
+    out = capsys.readouterr().out
+
+    assert result == 1
+    assert "does not match the file being diffed" in out
+    assert "foo.txt.orig." in out