diff --git a/src/presence.py b/src/presence.py
new file mode 100644
index 0000000..77607e0
--- /dev/null
+++ b/src/presence.py
@@ -0,0 +1,78 @@
+# This file is part of embargo_butler.
+#
+# Developed for the LSST Data Management System.
+# This product includes software developed by the LSST Project
+# (http://www.lsst.org).
+# See the COPYRIGHT file at the top-level directory of this distribution
+# for details of code ownership.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""
+Presence service to translate group names to image URIs.
+"""
+import os
+import re
+
+from flask import Flask
+
+from utils import setup_logging, setup_redis
+
+logger = setup_logging(__name__)
+r = setup_redis()
+
+# Delete image key when seen, before it expires, to save space
+delete_seen = os.environ.get("DELETE_SEEN") is not None
+
+app = Flask(__name__)
+
+
+@app.get("/presence/<instrument>/<group_name>/<int:snap_index>/<detector_name>")
+def presence(instrument: str, group_name: str, snap_index: int, detector_name: str) -> dict | tuple:
+    """Return the presence and URI of an image matching the parameters.
+
+    Parameters
+    ----------
+    instrument: `str`
+        Name of the instrument taking the image.
+    group_name: `str`
+        Name of the group (from the GROUPID FITS header).
+    snap_index: `int`
+        Number of the snap (zero-based).
+    detector_name: `str`
+        Name of the detector ("RNN_SNN").
+
+    Returns
+    -------
+    json: `dict`
+        JSON with "error", "present", "uri", and/or "message" keys.
+    """
+
+    try:
+        if instrument not in ("LATISS", "LSSTComCam", "LSSTComCamSim", "LSSTCam", "LSST-TS8"):
+            return ({"error": True, "message": f"Unknown instrument {instrument}"}, 400)
+        if not re.match(r"R\d\d_S\d\d", detector_name):
+            return ({"error": True, "message": f"Unrecognized detector name {detector_name}"}, 400)
+        key = f"GROUP:{instrument}:{group_name}:{snap_index}:{detector_name}"
+        result = r.get(key)
+        if result:
+            logger.info(f"Found key {key}")
+            if delete_seen:
+                r.delete(key)
+            return {"error": False, "present": True, "uri": result.decode()}
+        else:
+            logger.debug(f"No key {key}")
+            return {"error": False, "present": False}
+    except Exception as e:
+        return ({"error": True, "message": str(e)}, 500)