diff --git a/lib/crypto/OlmDevice.js b/lib/crypto/OlmDevice.js
index ca9565fe3..5d2fe6620 100644
--- a/lib/crypto/OlmDevice.js
+++ b/lib/crypto/OlmDevice.js
@@ -539,7 +539,9 @@ OlmDevice.prototype._saveInboundGroupSession = function(
  * @param {string} senderKey
  * @param {string} sessionId
  * @param {function} func
- * @return {object} result of func
+ * @return {object} Object with two keys "result": result of func, "exists"
+ *  whether the session exists. if the session doesn't exist then the function
+ *  isn't called and the "result" is undefined.
  * @private
  */
 OlmDevice.prototype._getInboundGroupSession = function(
@@ -550,7 +552,7 @@ OlmDevice.prototype._getInboundGroupSession = function(
     );
 
     if (r === null) {
-        throw new Error("Unknown inbound group session id");
+        return {sessionExists: false}
     }
 
     r = JSON.parse(r);
@@ -567,7 +569,7 @@ OlmDevice.prototype._getInboundGroupSession = function(
     var session = new Olm.InboundGroupSession();
     try {
         session.unpickle(this._pickleKey, r.session);
-        return func(session);
+        return {sessionExists: true, result: func(session)};
     } finally {
         session.free();
     }
@@ -603,7 +605,7 @@ OlmDevice.prototype.addInboundGroupSession = function(
  * @param {string} sessionId session identifier
  * @param {string} body      base64-encoded body of the encrypted message
  *
- * @return {string} plaintext
+ * @return {object} {result: "plaintext"|undefined, sessionExists: Boolean}
  */
 OlmDevice.prototype.decryptGroupMessage = function(
     roomId, senderKey, sessionId, body
diff --git a/lib/crypto/algorithms/megolm.js b/lib/crypto/algorithms/megolm.js
index b10794524..56d7bf226 100644
--- a/lib/crypto/algorithms/megolm.js
+++ b/lib/crypto/algorithms/megolm.js
@@ -356,7 +356,8 @@ utils.inherits(MegolmDecryption, base.DecryptionAlgorithm);
  *
  * @param {object} event raw event
  *
- * @return {object} decrypted payload (with properties 'type', 'content')
+ * @return {object} object with 'result' key with decrypted payload (with
+ *  properties 'type', 'content') and a 'sessionKey' key.
  *
  * @throws {module:crypto/algorithms/base.DecryptionError} if there is a
  *   problem decrypting the event
@@ -377,7 +378,11 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
         var res = this._olmDevice.decryptGroupMessage(
             event.room_id, content.sender_key, content.session_id, content.ciphertext
         );
-        return JSON.parse(res);
+        if (res.sessionExists) {
+            return {result: JSON.parse(res.result), sessionExists: true};
+        } else {
+            return {sessionExists: false};
+        }
     } catch (e) {
         throw new base.DecryptionError(e);
     }
diff --git a/lib/crypto/algorithms/olm.js b/lib/crypto/algorithms/olm.js
index d5fd4fd4e..e75877cdf 100644
--- a/lib/crypto/algorithms/olm.js
+++ b/lib/crypto/algorithms/olm.js
@@ -142,7 +142,9 @@ utils.inherits(OlmDecryption, base.DecryptionAlgorithm);
  *
  * @param {object} event raw event
  *
- * @return {object} decrypted payload (with properties 'type', 'content')
+ * @return {object} result object with result property with the decrypted
+ *  payload (with properties 'type', 'content'), and a "sessionExists" key
+ *  always set to true.
  *
  * @throws {module:crypto/algorithms/base.DecryptionError} if there is a
  *   problem decrypting the event
@@ -198,7 +200,7 @@ OlmDecryption.prototype.decryptEvent = function(event) {
     // TODO: Check the sender user id matches the sender key.
     // TODO: check the room_id and fingerprint
     if (payloadString !== null) {
-        return JSON.parse(payloadString);
+        return {result: JSON.parse(payloadString), sessionExists: true};
     } else {
         throw new base.DecryptionError("Bad Encrypted Message");
     }
diff --git a/lib/crypto/index.js b/lib/crypto/index.js
index f5184c40b..00284064d 100644
--- a/lib/crypto/index.js
+++ b/lib/crypto/index.js
@@ -820,7 +820,41 @@ Crypto.prototype.decryptEvent = function(event) {
     var alg = new AlgClass({
         olmDevice: this._olmDevice,
     });
-    return alg.decryptEvent(event);
+    var r = alg.decryptEvent(event);
+    if (r.sessionExists) {
+        return r.result;
+    } else {
+        // We've got a message for a session we don't have.
+        // Maybe the sender forgot to tell us about the session.
+        // Remind the sender that we exists so that they might
+        // tell us about the sender.
+        if (event.getRoomId !== undefined && event.getSender !== undefined) {
+            var senderUserId = event.getSender();
+            var roomId = event.getRoomId();
+            var content = {};
+            var senderDeviceId = event.content.device_id;
+            if (senderDeviceId !== undefined) {
+                content[senderUserId][senderDeviceId] = {
+                    device_id: this._deviceId,
+                    rooms: [roomId],
+                };
+            } else {
+                content[senderUserId]["*"] = {
+                    device_id: this._deviceId,
+                    rooms: [roomId],
+                };
+            }
+            // TODO: Ratelimit the "m.new_device" messages to make sure we don't
+            // flood the target device with messages if we get lots of encrypted
+            // messages from them at once.
+            this._baseApis.sendToDevice(
+                "m.new_device", // OH HAI!
+                content
+            ).done(function() {});
+        }
+
+        throw new algorithms.DecryptionError("Unknown inbound session id");
+    }
 };
 
 /**