diff --git a/index.html b/index.html
index d049580..06cff1c 100644
--- a/index.html
+++ b/index.html
@@ -983,12 +983,60 @@ Sovereignty and service always.`;
 
 
   // ===== OVERLAY =====
+
+  // Focus trap: cycle through focusable elements within the crisis overlay
+  function getOverlayFocusableElements() {
+    return crisisOverlay.querySelectorAll(
+      'a[href], button:not([disabled]), [tabindex]:not([tabindex="-1"])'
+    );
+  }
+
+  function trapFocusInOverlay(e) {
+    if (!crisisOverlay.classList.contains('active')) return;
+    if (e.key !== 'Tab') return;
+
+    var focusable = getOverlayFocusableElements();
+    if (focusable.length === 0) return;
+
+    var first = focusable[0];
+    var last = focusable[focusable.length - 1];
+
+    if (e.shiftKey) {
+      // Shift+Tab: if on first, wrap to last
+      if (document.activeElement === first) {
+        e.preventDefault();
+        last.focus();
+      }
+    } else {
+      // Tab: if on last, wrap to first
+      if (document.activeElement === last) {
+        e.preventDefault();
+        first.focus();
+      }
+    }
+  }
+
+  // Store the element that had focus before the overlay opened
+  var _preOverlayFocusElement = null;
+
   function showOverlay() {
+    // Save current focus for restoration on dismiss
+    _preOverlayFocusElement = document.activeElement;
+
     crisisOverlay.classList.add('active');
     overlayDismissBtn.disabled = true;
     var countdown = 10;
     overlayDismissBtn.textContent = 'Continue to chat (' + countdown + 's)';
 
+    // Disable background interaction via inert attribute
+    var mainApp = document.querySelector('.app');
+    if (mainApp) mainApp.setAttribute('inert', '');
+    // Also hide from assistive tech
+    var chatSection = document.getElementById('chat');
+    if (chatSection) chatSection.setAttribute('aria-hidden', 'true');
+    var footerEl = document.querySelector('footer');
+    if (footerEl) footerEl.setAttribute('aria-hidden', 'true');
+
     if (overlayTimer) clearInterval(overlayTimer);
     overlayTimer = setInterval(function() {
       countdown--;
@@ -1005,6 +1053,9 @@ Sovereignty and service always.`;
     overlayDismissBtn.focus();
   }
 
+  // Register focus trap on document (always listening, gated by class check)
+  document.addEventListener('keydown', trapFocusInOverlay);
+
   overlayDismissBtn.addEventListener('click', function() {
     if (!overlayDismissBtn.disabled) {
       crisisOverlay.classList.remove('active');
@@ -1012,7 +1063,22 @@ Sovereignty and service always.`;
         clearInterval(overlayTimer);
         overlayTimer = null;
       }
-      msgInput.focus();
+
+      // Re-enable background interaction
+      var mainApp = document.querySelector('.app');
+      if (mainApp) mainApp.removeAttribute('inert');
+      var chatSection = document.getElementById('chat');
+      if (chatSection) chatSection.removeAttribute('aria-hidden');
+      var footerEl = document.querySelector('footer');
+      if (footerEl) footerEl.removeAttribute('aria-hidden');
+
+      // Restore focus to the element that had it before the overlay opened
+      if (_preOverlayFocusElement && typeof _preOverlayFocusElement.focus === 'function') {
+        _preOverlayFocusElement.focus();
+      } else {
+        msgInput.focus();
+      }
+      _preOverlayFocusElement = null;
     }
   });
 
diff --git a/tests/test_crisis_overlay_focus_trap.py b/tests/test_crisis_overlay_focus_trap.py
new file mode 100644
index 0000000..f657afc
--- /dev/null
+++ b/tests/test_crisis_overlay_focus_trap.py
@@ -0,0 +1,57 @@
+import pathlib
+import re
+import unittest
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+INDEX_HTML = ROOT / 'index.html'
+
+
+class TestCrisisOverlayFocusTrap(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.html = INDEX_HTML.read_text()
+
+    def test_overlay_registers_tab_key_focus_trap(self):
+        self.assertRegex(
+            self.html,
+            r"function\s+trapFocusInOverlay\s*\(e\)",
+            'Expected crisis overlay focus trap handler to exist.',
+        )
+        self.assertRegex(
+            self.html,
+            r"if\s*\(e\.key\s*!==\s*'Tab'\)\s*return;",
+            'Expected focus trap handler to guard on Tab key events.',
+        )
+        self.assertRegex(
+            self.html,
+            r"document\.addEventListener\('keydown',\s*trapFocusInOverlay\)",
+            'Expected overlay focus trap to register on document keydown.',
+        )
+
+    def test_overlay_disables_background_interaction(self):
+        self.assertRegex(
+            self.html,
+            r"mainApp\.setAttribute\('inert',\s*''\)",
+            'Expected overlay to set inert on the main app while active.',
+        )
+        self.assertRegex(
+            self.html,
+            r"mainApp\.removeAttribute\('inert'\)",
+            'Expected overlay dismissal to remove inert from the main app.',
+        )
+
+    def test_overlay_restores_focus_after_dismiss(self):
+        self.assertRegex(
+            self.html,
+            r"_preOverlayFocusElement\s*=\s*document\.activeElement",
+            'Expected overlay to remember the pre-overlay focus target.',
+        )
+        self.assertRegex(
+            self.html,
+            r"_preOverlayFocusElement\.focus\(\)",
+            'Expected overlay dismissal to restore focus to the prior target.',
+        )
+
+
+if __name__ == '__main__':
+    unittest.main()