diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1008,6 +1008,7 @@
     'HarbormasterQueryBuildablesConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterQueryBuildablesConduitAPIMethod.php',
     'HarbormasterQueryBuildsConduitAPIMethod' => 'applications/harbormaster/conduit/HarbormasterQueryBuildsConduitAPIMethod.php',
     'HarbormasterRemarkupRule' => 'applications/harbormaster/remarkup/HarbormasterRemarkupRule.php',
+    'HarbormasterRunBuildPlanBuildImplementation' => 'applications/harbormaster/step/HarbormasterRunBuildPlanBuildImplementation.php',
     'HarbormasterRunBuildPlansHeraldAction' => 'applications/harbormaster/herald/HarbormasterRunBuildPlansHeraldAction.php',
     'HarbormasterSchemaSpec' => 'applications/harbormaster/storage/HarbormasterSchemaSpec.php',
     'HarbormasterScratchTable' => 'applications/harbormaster/storage/HarbormasterScratchTable.php',
@@ -4743,6 +4744,7 @@
     'HarbormasterQueryBuildablesConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterQueryBuildsConduitAPIMethod' => 'HarbormasterConduitAPIMethod',
     'HarbormasterRemarkupRule' => 'PhabricatorObjectRemarkupRule',
+    'HarbormasterRunBuildPlanBuildImplementation' => 'HarbormasterBuildStepImplementation',
     'HarbormasterRunBuildPlansHeraldAction' => 'HeraldAction',
     'HarbormasterSchemaSpec' => 'PhabricatorConfigSchemaSpec',
     'HarbormasterScratchTable' => 'HarbormasterDAO',
diff --git a/src/applications/harbormaster/step/HarbormasterRunBuildPlanBuildImplementation.php b/src/applications/harbormaster/step/HarbormasterRunBuildPlanBuildImplementation.php
new file mode 100644
--- /dev/null
+++ b/src/applications/harbormaster/step/HarbormasterRunBuildPlanBuildImplementation.php
@@ -0,0 +1,170 @@
+<?php
+
+final class HarbormasterRunBuildPlanBuildImplementation
+  extends HarbormasterBuildStepImplementation {
+
+  public function getName() {
+    return pht('Start or Wait for Build Plan');
+  }
+
+  public function getGenericDescription() {
+    return pht(
+      'Start or wait for another build plan to finish on the same buildable.');
+  }
+
+  public function getDescription() {
+    $target_plan_id = $this->getSetting('id');
+
+    $target_plan = id(new HarbormasterBuildPlanQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withIDs(array($target_plan_id))
+      ->executeOne();
+
+    if ($target_plan === null) {
+      return pht(
+        'Start and / or wait for a non-existent build plan '.
+        'to finish on this buildable.');
+    }
+
+    $name = $this->formatValueForDescription($target_plan->getName());
+
+    return pht(
+      'Start and / or wait for build plan %s to finish on this buildable.',
+      $name);
+  }
+
+  public function execute(
+    HarbormasterBuild $build,
+    HarbormasterBuildTarget $build_target) {
+
+    $target_plan_id = $this->getSetting('id');
+
+    // Get the target build plan type.
+    $target_plan = id(new HarbormasterBuildPlanQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withIDs(array($target_plan_id))
+      ->executeOne();
+    if (!$target_plan) {
+      throw new Exception('Unable to find build plan with ID '.$target_plan_id);
+    }
+
+    // If the target plan is the same as the current plan, we
+    // automatically pass.
+    if ($target_plan->getPHID() === $build->getBuildPlanPHID()) {
+      return;
+    }
+
+    // Find all other builds running on this buildable.
+    $buildable = $build->getBuildable();
+    $other_builds = id(new HarbormasterBuildQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withBuildablePHIDs(array($buildable->getPHID()))
+      ->execute();
+
+    // Find the build for that build plan (if it exists).
+    $target_build = null;
+    foreach ($other_builds as $other_build) {
+      if ($other_build->getBuildPlanPHID() === $target_plan->getPHID()) {
+        $target_build = $other_build;
+        break;
+      }
+    }
+
+    if ($target_build === null) {
+      // There is no current build with this build plan on the buildable,
+      // so now we're going to start one.
+      $target_build = $build->getBuildable()->applyPlan($target_plan);
+    } else {
+      // If the build plan has failed on a previous run, restart it.
+      switch ($target_build->getBuildStatus()) {
+        case HarbormasterBuild::STATUS_FAILED:
+        case HarbormasterBuild::STATUS_ERROR: {
+          $harbormaster_phid = id(new PhabricatorHarbormasterApplication())
+            ->getPHID();
+
+          $daemon_source = PhabricatorContentSource::newForSource(
+            PhabricatorContentSource::SOURCE_DAEMON,
+            array());
+
+          $editor = id(new HarbormasterBuildTransactionEditor())
+            ->setActor(PhabricatorUser::getOmnipotentUser())
+            ->setActingAsPHID($harbormaster_phid)
+            ->setContentSource($daemon_source)
+            ->setContinueOnNoEffect(true)
+            ->setContinueOnMissingFields(true);
+
+          $xaction = id(new HarbormasterBuildTransaction())
+            ->setTransactionType(HarbormasterBuildTransaction::TYPE_COMMAND)
+            ->setNewValue(HarbormasterBuildCommand::COMMAND_RESTART);
+
+          $editor->applyTransactions($target_build, array($xaction));
+
+          break;
+        }
+      }
+    }
+
+    $artifact = $build->createArtifact(
+      $build_target,
+      'build_plan_'.$build_target->getID(),
+      HarbormasterBuildArtifact::TYPE_URI);
+    $artifact->setArtifactData(array(
+      'name' => 'Target Build Plan',
+      'uri' => '/harbormaster/plan/'.$target_plan->getID(),
+    ));
+    $artifact->save();
+
+    $artifact = $build->createArtifact(
+      $build_target,
+      'build_'.$build_target->getID(),
+      HarbormasterBuildArtifact::TYPE_URI);
+    $artifact->setArtifactData(array(
+      'name' => 'Target Build',
+      'uri' => '/harbormaster/build/'.$target_build->getID(),
+    ));
+    $artifact->save();
+
+    $target_build_phid = $target_build->getPHID();
+
+    do {
+      $target_build = id(new HarbormasterBuildQuery())
+        ->setViewer(PhabricatorUser::getOmnipotentUser())
+        ->withPHIDs(array($target_build_phid))
+        ->executeOne();
+
+      if (!$target_build) {
+        throw new Exception('Build has disappeared!');
+      }
+
+      // If the build has not yet passed, yield for the moment.
+      if ($target_build->isBuilding() || $target_build->isRestarting() ||
+        $target_build->getBuildStatus() === HarbormasterBuild::STATUS_STOPPED) {
+        sleep(15);
+        continue;
+      }
+
+      // Otherwise check the result and if it's an error or failure result,
+      // then throw an exception.
+      switch ($target_build->getBuildStatus()) {
+        case HarbormasterBuild::STATUS_FAILED:
+        case HarbormasterBuild::STATUS_ERROR:
+          throw new HarbormasterBuildFailureException();
+      }
+
+      // Pass if we get to here.
+      break;
+    } while (true);
+  }
+
+  public function getFieldSpecifications() {
+    return array(
+      'id' => array(
+        'name' => pht('Target Build Plan ID'),
+        'type' => 'text',
+        'required' => true,
+      ),
+    );
+  }
+
+
+}