diff --git a/lib/worker.js b/lib/worker.js
index 60072a68fc..f32aef648e 100644
--- a/lib/worker.js
+++ b/lib/worker.js
@@ -1,63 +1,99 @@
 'use strict';
 
+const {createInvalidArgumentTypeError} = require('./errors');
 const workerpool = require('workerpool');
 const Mocha = require('./mocha');
 const {handleRequires, validatePlugin} = require('./cli/run-helpers');
-const debug = require('debug')('mocha:worker');
+const debug = require('debug')(`mocha:worker:${process.pid}`);
 const {serialize} = require('./serializer');
-let bootstrapped = false;
+
+const BUFFERED_REPORTER_PATH = require.resolve('./reporters/buffered');
+
+if (workerpool.isMainThread) {
+  throw new Error(
+    'This script is intended to be run as a worker (by the `workerpool` package).'
+  );
+}
+
+/**
+ * Initializes some stuff on the first call to {@link run}.
+ *
+ * Handles `--require` and `--ui`.  Does _not_ handle `--reporter`,
+ * as only the `Buffered` reporter is used.
+ *
+ * **This function only runs once**; it overwrites itself with a no-op
+ * before returning.
+ *
+ * @param {Options} argv - Command-line options
+ */
+let bootstrap = argv => {
+  handleRequires(argv.require);
+  validatePlugin(argv, 'ui', Mocha.interfaces);
+  process.on('beforeExit', () => {
+    /* istanbul ignore next */
+    debug('exiting');
+  });
+  debug('bootstrapped');
+  bootstrap = () => {};
+};
 
 /**
  * Runs a single test file in a worker thread.
- * @param {string} file - Filepath of test file
- * @param {Options} argv - Parsed command-line options object
- * @returns {Promise<[number, BufferedEvent[]]>} A tuple of failures and
- * serializable event data
+ * @param {string} filepath - Filepath of test file
+ * @param {Options} [argv] - Parsed command-line options object
+ * @returns {Promise<{failures: number, events: BufferedEvent[]}>} - Test
+ * failure count and list of events.
  */
-async function run(file, argv) {
-  debug('running test file %s on process [%d]', file, process.pid);
-  // the buffered reporter retains its events; these events are returned
-  // from this function back to the main process.
-  argv.reporter = require.resolve('./reporters/buffered');
-  // if these were set, it would cause infinite recursion by spawning another worker
-  delete argv.parallel;
-  delete argv.jobs;
-  if (!bootstrapped) {
-    // setup requires and ui, but only do this once--we will reuse this worker!
-    handleRequires(argv.require);
-    validatePlugin(argv, 'ui', Mocha.interfaces);
-    bootstrapped = true;
-    debug('bootstrapped process [%d]', process.pid);
+async function run(filepath, argv = {ui: 'bdd'}) {
+  if (!filepath) {
+    throw createInvalidArgumentTypeError(
+      'Expected a non-empty "filepath" argument',
+      'file',
+      'string'
+    );
   }
-  const mocha = new Mocha(argv);
-  mocha.files = [file];
+
+  debug('running test file %s', filepath);
+
+  const opts = Object.assign(argv, {
+    // workers only use the `Buffered` reporter.
+    reporter: BUFFERED_REPORTER_PATH,
+    // if this was true, it would cause infinite recursion.
+    parallel: false
+  });
+
+  bootstrap(opts);
+
+  const mocha = new Mocha(opts).addFile(filepath);
+
   try {
     await mocha.loadFilesAsync();
   } catch (err) {
-    debug(
-      'process [%d] rejecting; could not load file %s: %s',
-      process.pid,
-      file,
-      err
-    );
+    debug('could not load file %s: %s', filepath, err);
     throw err;
   }
-  return new Promise(resolve => {
-    // TODO: figure out exactly what the sad path looks like here.
-    // will depend on allowUncaught
-    // rejection should only happen if an error is "unrecoverable"
+
+  return new Promise((resolve, reject) => {
     mocha.run(result => {
+      // Runner adds these; if we don't remove them, we'll get a leak.
       process.removeAllListeners('uncaughtException');
-      debug('process [%d] resolving', process.pid);
-      resolve(serialize(result));
+
+      debug('completed run with %d test failures', result.failures);
+      try {
+        resolve(serialize(result));
+      } catch (err) {
+        // TODO: figure out exactly what the sad path looks like here.
+        // rejection should only happen if an error is "unrecoverable"
+        reject(err);
+      }
     });
   });
 }
 
-workerpool.worker({
-  run
-});
+// this registers the `run` function.
+workerpool.worker({run});
+
+debug('running');
 
-process.on('beforeExit', () => {
-  debug('process [%d] exiting', process.pid);
-});
+// for testing
+exports.run = run;
diff --git a/test/node-unit/worker.spec.js b/test/node-unit/worker.spec.js
new file mode 100644
index 0000000000..4b62a1f9b6
--- /dev/null
+++ b/test/node-unit/worker.spec.js
@@ -0,0 +1,186 @@
+'use strict';
+
+const {SerializableWorkerResult} = require('../../lib/serializer');
+const rewiremock = require('rewiremock/node');
+const {createSandbox} = require('sinon');
+
+const WORKER_PATH = require.resolve('../../lib/worker.js');
+
+describe('worker', function() {
+  let worker;
+  let workerpoolWorker;
+  let sandbox;
+
+  beforeEach(function() {
+    sandbox = createSandbox();
+    workerpoolWorker = sandbox.stub();
+    sandbox.spy(process, 'removeAllListeners');
+  });
+
+  describe('when run as main "thread"', function() {
+    it('should throw', function() {
+      expect(() => {
+        rewiremock.proxy(WORKER_PATH, {
+          workerpool: {
+            isMainThread: true,
+            worker: workerpoolWorker
+          }
+        });
+      }, 'to throw');
+    });
+  });
+
+  describe('when run as "worker thread"', function() {
+    class MockMocha {}
+    let serializer;
+    let runHelpers;
+
+    beforeEach(function() {
+      MockMocha.prototype.addFile = sandbox.stub().returnsThis();
+      MockMocha.prototype.loadFilesAsync = sandbox.stub();
+      MockMocha.prototype.run = sandbox.stub();
+      MockMocha.interfaces = {
+        bdd: sandbox.stub()
+      };
+
+      serializer = {
+        serialize: sandbox.stub()
+      };
+
+      runHelpers = {
+        handleRequires: sandbox.stub(),
+        validatePlugin: sandbox.stub()
+      };
+
+      worker = rewiremock.proxy(WORKER_PATH, {
+        workerpool: {
+          isMainThread: false,
+          worker: workerpoolWorker
+        },
+        '../../lib/mocha': MockMocha,
+        '../../lib/serializer': serializer,
+        '../../lib/cli/run-helpers': runHelpers
+      });
+    });
+
+    it('should register itself with workerpool', function() {
+      expect(workerpoolWorker, 'to have a call satisfying', [
+        {run: worker.run}
+      ]);
+    });
+
+    describe('function', function() {
+      describe('run', function() {
+        describe('when called without arguments', function() {
+          it('should reject', async function() {
+            return expect(worker.run, 'to be rejected with error satisfying', {
+              code: 'ERR_MOCHA_INVALID_ARG_TYPE'
+            });
+          });
+        });
+
+        describe('when called with empty "filepath" argument', function() {
+          it('should reject', async function() {
+            return expect(
+              () => worker.run(''),
+              'to be rejected with error satisfying',
+              {
+                code: 'ERR_MOCHA_INVALID_ARG_TYPE'
+              }
+            );
+          });
+        });
+
+        describe('when the file at "filepath" argument is unloadable', function() {
+          it('should reject', async function() {
+            MockMocha.prototype.loadFilesAsync.rejects();
+            return expect(
+              () => worker.run('some-non-existent-file.js'),
+              'to be rejected'
+            );
+          });
+        });
+
+        describe('when the file at "filepath" is loadable', function() {
+          let result;
+          beforeEach(function() {
+            result = SerializableWorkerResult.create();
+
+            MockMocha.prototype.loadFilesAsync.resolves();
+            MockMocha.prototype.run.yields(result);
+          });
+
+          it('should handle "--require"', async function() {
+            await worker.run('some-file.js', {require: 'foo'});
+            expect(runHelpers.handleRequires, 'to have a call satisfying', [
+              'foo'
+            ]).and('was called once');
+          });
+
+          it('should handle "--ui"', async function() {
+            const argv = {};
+            await worker.run('some-file.js', argv);
+
+            expect(runHelpers.validatePlugin, 'to have a call satisfying', [
+              argv,
+              'ui',
+              MockMocha.interfaces
+            ]).and('was called once');
+          });
+
+          it('should call Mocha#run', async function() {
+            await worker.run('some-file.js');
+            expect(MockMocha.prototype.run, 'was called once');
+          });
+
+          it('should remove all uncaughtException listeners', async function() {
+            await worker.run('some-file.js');
+            expect(process.removeAllListeners, 'to have a call satisfying', [
+              'uncaughtException'
+            ]);
+          });
+
+          describe('when serialization succeeds', function() {
+            beforeEach(function() {
+              serializer.serialize.returnsArg(0);
+            });
+
+            it('should resolve with a SerializedWorkerResult', async function() {
+              return expect(
+                worker.run('some-file.js'),
+                'to be fulfilled with',
+                result
+              );
+            });
+          });
+
+          describe('when serialization fails', function() {
+            beforeEach(function() {
+              serializer.serialize.throws();
+            });
+
+            it('should reject', async function() {
+              return expect(worker.run('some-file.js'), 'to be rejected');
+            });
+          });
+
+          describe('when run twice', function() {
+            it('should initialize only once', async function() {
+              await worker.run('some-file.js');
+              await worker.run('some-other-file.js');
+
+              expect(runHelpers, 'to satisfy', {
+                handleRequires: expect.it('was called once'),
+                validatePlugin: expect.it('was called once')
+              });
+            });
+          });
+        });
+      });
+    });
+  });
+
+  afterEach(function() {
+    sandbox.restore();
+  });
+});