diff --git a/tensorboard/backend/event_processing/plugin_event_accumulator.py b/tensorboard/backend/event_processing/plugin_event_accumulator.py
index afa2f4d420..7e2bb206f7 100644
--- a/tensorboard/backend/event_processing/plugin_event_accumulator.py
+++ b/tensorboard/backend/event_processing/plugin_event_accumulator.py
@@ -159,6 +159,7 @@ def __init__(
         self._generator_mutex = threading.Lock()
 
         self.purge_orphaned_data = purge_orphaned_data
+        self._seen_session_start = False
 
         self.most_recent_step = -1
         self.most_recent_wall_time = -1
@@ -510,11 +511,16 @@ def _MaybePurgeOrphanedData(self, event):
     def _CheckForRestartAndMaybePurge(self, event):
         """Check and discard expired events using SessionLog.START.
 
-        Check for a SessionLog.START event and purge all previously seen events
-        with larger steps, because they are out of date. Because of supervisor
-        threading, it is possible that this logic will cause the first few event
-        messages to be discarded since supervisor threading does not guarantee
-        that the START message is deterministically written first.
+        The first SessionLog.START event in a run indicates the start of a
+        supervisor session. Subsequent SessionLog.START events indicate a
+        *restart*, which may need to preempt old events. This method checks
+        for a session restart event and purges all previously seen events whose
+        step is larger than or equal to this event's step.
+
+        Because of supervisor threading, it is possible that this logic will
+        cause the first few event messages to be discarded since supervisor
+        threading does not guarantee that the START message is deterministically
+        written first.
 
         This method is preferred over _CheckForOutOfOrderStepAndMaybePurge which
         can inadvertently discard events due to supervisor threading.
@@ -523,11 +529,13 @@ def _CheckForRestartAndMaybePurge(self, event):
           event: The event to use as reference. If the event is a START event, all
             previously seen events with a greater event.step will be purged.
         """
-        if (
-            event.HasField("session_log")
-            and event.session_log.status == event_pb2.SessionLog.START
-        ):
-            self._Purge(event, by_tags=False)
+        if event.session_log.status != event_pb2.SessionLog.START:
+            return
+        if not self._seen_session_start:
+            # Initial start event: does not indicate a restart.
+            self._seen_session_start = True
+            return
+        self._Purge(event, by_tags=False)
 
     def _CheckForOutOfOrderStepAndMaybePurge(self, event):
         """Check for out-of-order event.step and discard expired events for
diff --git a/tensorboard/backend/event_processing/plugin_event_accumulator_test.py b/tensorboard/backend/event_processing/plugin_event_accumulator_test.py
index eb2ffb6b88..e6178480a4 100644
--- a/tensorboard/backend/event_processing/plugin_event_accumulator_test.py
+++ b/tensorboard/backend/event_processing/plugin_event_accumulator_test.py
@@ -326,11 +326,14 @@ def testSessionLogStartMessageDiscardsExpiredEvents(self):
         """
         gen = _EventGenerator(self)
         acc = ea.EventAccumulator(gen)
+        slog = event_pb2.SessionLog(status=event_pb2.SessionLog.START)
+
         gen.AddEvent(
             event_pb2.Event(wall_time=0, step=1, file_version="brain.Event:2")
         )
 
         gen.AddScalarTensor("s1", wall_time=1, step=100, value=20)
+        gen.AddEvent(event_pb2.Event(wall_time=1, step=100, session_log=slog))
         gen.AddScalarTensor("s1", wall_time=1, step=200, value=20)
         gen.AddScalarTensor("s1", wall_time=1, step=300, value=20)
         gen.AddScalarTensor("s1", wall_time=1, step=400, value=20)
@@ -338,7 +341,6 @@ def testSessionLogStartMessageDiscardsExpiredEvents(self):
         gen.AddScalarTensor("s2", wall_time=1, step=202, value=20)
         gen.AddScalarTensor("s2", wall_time=1, step=203, value=20)
 
-        slog = event_pb2.SessionLog(status=event_pb2.SessionLog.START)
         gen.AddEvent(event_pb2.Event(wall_time=2, step=201, session_log=slog))
         acc.Reload()
         self.assertEqual([x.step for x in acc.Tensors("s1")], [100, 200])