diff --git a/includes/Checker/Preparations/Force_Single_Plugin_Preparation.php b/includes/Checker/Preparations/Force_Single_Plugin_Preparation.php
new file mode 100644
index 000000000..5041b283d
--- /dev/null
+++ b/includes/Checker/Preparations/Force_Single_Plugin_Preparation.php
@@ -0,0 +1,103 @@
+<?php
+/**
+ * Class WordPress\Plugin_Check\Checker\Preparations\Force_Single_Plugin_Preparation
+ *
+ * @package plugin-check
+ */
+
+namespace WordPress\Plugin_Check\Checker\Preparations;
+
+use WordPress\Plugin_Check\Checker\Preparation;
+use Exception;
+
+/**
+ * Class for the preparation to force the plugin to be checked as the only active plugin.
+ *
+ * This ensures the plugin is checked as much in isolation as possible.
+ *
+ * @since n.e.x.t
+ */
+class Force_Single_Plugin_Preparation implements Preparation {
+
+	/**
+	 * Plugin slug.
+	 *
+	 * @since n.e.x.t
+	 * @var string
+	 */
+	protected $plugin_basename;
+
+	/**
+	 * Sets the plugin slug.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $plugin_basename Slug of the plugin, E.g. "akismet\akismet.php".
+	 */
+	public function __construct( $plugin_basename ) {
+		$this->plugin_basename = $plugin_basename;
+	}
+
+	/**
+	 * Runs this preparation step for the environment and returns a cleanup function.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return callable Cleanup function to revert any changes made here.
+	 *
+	 * @throws Exception Thrown when preparation fails.
+	 */
+	public function prepare() {
+		$valid_plugin = validate_plugin( $this->plugin_basename );
+
+		// Check if the plugin exists.
+		if ( is_wp_error( $valid_plugin ) ) {
+
+			throw new Exception(
+				sprintf(
+					/* translators: 1: plugin basename, 2: error message */
+					__( 'Invalid plugin %1$s: %2$s', 'plugin-check' ),
+					$this->plugin_basename,
+					$valid_plugin->get_error_message()
+				)
+			);
+		}
+
+		add_filter( 'option_active_plugins', array( $this, 'filter_active_plugins' ) );
+		add_filter( 'default_option_active_plugins', array( $this, 'filter_active_plugins' ) );
+
+		// Return the cleanup function.
+		return function() {
+			remove_filter( 'option_active_plugins', array( $this, 'filter_active_plugins' ) );
+			remove_filter( 'default_option_active_plugins', array( $this, 'filter_active_plugins' ) );
+		};
+	}
+
+	/**
+	 * Filter active plugins.
+	 *
+	 * @param array $active_plugins List of active plugins.
+	 * @return array List of active plugins.
+	 */
+	public function filter_active_plugins( $active_plugins ) {
+		if ( is_array( $active_plugins ) && in_array( $this->plugin_basename, $active_plugins, true ) ) {
+
+			$plugin_base_file = plugin_basename( WP_PLUGIN_CHECK_MAIN_FILE );
+
+			// If the plugin-check is the only available plugin then return that one only.
+			if ( $this->plugin_basename === $plugin_base_file ) {
+
+				return array(
+					$plugin_base_file,
+				);
+			}
+
+			return array(
+				$this->plugin_basename,
+				$plugin_base_file,
+			);
+		}
+
+		return $active_plugins;
+	}
+}
diff --git a/plugin-check.php b/plugin-check.php
index 685f752e5..5c31b12b2 100644
--- a/plugin-check.php
+++ b/plugin-check.php
@@ -17,7 +17,8 @@
 
 define( 'WP_PLUGIN_CHECK_VERSION', 'n.e.x.t' );
 define( 'WP_PLUGIN_CHECK_MINIMUM_PHP', '5.6' );
-define( 'WP_PLUGIN_CHECK_PLUGIN_DIR_PATH', plugin_dir_path( __FILE__ ) );
+define( 'WP_PLUGIN_CHECK_MAIN_FILE', __FILE__ );
+define( 'WP_PLUGIN_CHECK_PLUGIN_DIR_PATH', plugin_dir_path( WP_PLUGIN_CHECK_MAIN_FILE ) );
 
 /**
  * Checks basic requirements and loads the plugin.
@@ -42,7 +43,7 @@ function wp_plugin_check_load() {
 
 	// Setup the plugin.
 	$class_name = 'WordPress\\Plugin_Check\\Plugin_Main';
-	$instance   = new $class_name( __FILE__ );
+	$instance   = new $class_name( WP_PLUGIN_CHECK_MAIN_FILE );
 	$instance->add_hooks();
 }
 
diff --git a/tests/Checker/Check_Context_Tests.php b/tests/Checker/Check_Context_Tests.php
index 1dfff4926..a5c893bdc 100644
--- a/tests/Checker/Check_Context_Tests.php
+++ b/tests/Checker/Check_Context_Tests.php
@@ -12,11 +12,11 @@ public function set_up() {
 		parent::set_up();
 
 		$this->plugin_name   = basename( TESTS_PLUGIN_DIR );
-		$this->check_context = new Check_Context( WP_PLUGIN_DIR . '/' . $this->plugin_name . '/plugin-check.php' );
+		$this->check_context = new Check_Context( WP_PLUGIN_CHECK_MAIN_FILE );
 	}
 
 	public function test_basename() {
-		$this->assertSame( $this->plugin_name . '/plugin-check.php', $this->check_context->basename() );
+		$this->assertSame( plugin_basename( WP_PLUGIN_CHECK_MAIN_FILE ), $this->check_context->basename() );
 	}
 
 	public function test_path() {
diff --git a/tests/Checker/Preparations/Force_Single_Plugin_Preparation_Tests.php b/tests/Checker/Preparations/Force_Single_Plugin_Preparation_Tests.php
new file mode 100644
index 000000000..52b7aa843
--- /dev/null
+++ b/tests/Checker/Preparations/Force_Single_Plugin_Preparation_Tests.php
@@ -0,0 +1,88 @@
+<?php
+/**
+ * Tests for the Force_Single_Plugin_Preparation class.
+ *
+ * @package plugin-check
+ */
+
+namespace Checker\Preparations;
+
+use WordPress\Plugin_Check\Checker\Preparations\Force_Single_Plugin_Preparation;
+use WP_UnitTestCase;
+use Exception;
+
+class Force_Single_Plugin_Preparation_Tests extends WP_UnitTestCase {
+
+	protected $plugin_basename_file;
+
+	public function set_up() {
+		parent::set_up();
+
+		$this->plugin_basename_file = plugin_basename( WP_PLUGIN_CHECK_MAIN_FILE );
+	}
+
+	public function test_prepare_plugin_exists() {
+
+		$preparation = new Force_Single_Plugin_Preparation( 'akismet/akismet.php' );
+
+		$this->expectException( 'Exception' );
+		$this->expectExceptionMessage( 'Invalid plugin akismet/akismet.php: Plugin file does not exist.' );
+		$preparation->prepare();
+	}
+
+	/**
+	 * @throws Exception Throw exception.
+	 */
+	public function test_prepare() {
+		// Remove the WP tests active plugins filter which interfers with this test.
+		remove_filter( 'pre_option_active_plugins', 'wp_tests_options' );
+
+		$preparation    = new Force_Single_Plugin_Preparation( $this->plugin_basename_file );
+		$active_plugins = array(
+			'akismet/akismet.php',
+			$this->plugin_basename_file,
+			'wp-reset/wp-reset.php',
+		);
+
+		update_option( 'active_plugins', $active_plugins );
+
+		$cleanup = $preparation->prepare();
+		$before  = get_option( 'active_plugins' );
+		$cleanup();
+		$after = get_option( 'active_plugins' );
+
+		$this->assertSame( array( $this->plugin_basename_file ), $before );
+		$this->assertSame( $active_plugins, $after );
+	}
+
+	public function test_filter_active_plugins() {
+
+		$preparation = new Force_Single_Plugin_Preparation( 'wp-reset/wp-reset.php' );
+
+		$plugins = array(
+			'akismet/akismet.php',
+			$this->plugin_basename_file,
+			'wp-reset/wp-reset.php',
+		);
+
+		$active_plugins = $preparation->filter_active_plugins( $plugins );
+
+		$this->assertSame(
+			array(
+				'wp-reset/wp-reset.php',
+				$this->plugin_basename_file,
+			),
+			$active_plugins
+		);
+
+		$plugins = array(
+			'akismet/akismet.php',
+			$this->plugin_basename_file,
+			'test-plugin/test-plugin.php',
+		);
+
+		$active_plugins = $preparation->filter_active_plugins( $plugins );
+
+		$this->assertSame( $plugins, $active_plugins );
+	}
+}
diff --git a/tests/Plugin_Context_Tests.php b/tests/Plugin_Context_Tests.php
index 74f8b1975..3a865d59c 100644
--- a/tests/Plugin_Context_Tests.php
+++ b/tests/Plugin_Context_Tests.php
@@ -12,11 +12,11 @@ public function set_up() {
 		parent::set_up();
 
 		$this->plugin_name    = basename( TESTS_PLUGIN_DIR );
-		$this->plugin_context = new Plugin_Context( WP_PLUGIN_DIR . '/' . $this->plugin_name . '/plugin-check.php' );
+		$this->plugin_context = new Plugin_Context( WP_PLUGIN_CHECK_MAIN_FILE );
 	}
 
 	public function test_basename() {
-		$this->assertSame( $this->plugin_name . '/plugin-check.php', $this->plugin_context->basename() );
+		$this->assertSame( plugin_basename( WP_PLUGIN_CHECK_MAIN_FILE ), $this->plugin_context->basename() );
 	}
 
 	public function test_path() {
diff --git a/tests/Plugin_Main_Tests.php b/tests/Plugin_Main_Tests.php
index 79f472541..755421dac 100644
--- a/tests/Plugin_Main_Tests.php
+++ b/tests/Plugin_Main_Tests.php
@@ -12,7 +12,7 @@ class Plugin_Main_Tests extends WP_UnitTestCase {
 	public function set_up() {
 		parent::set_up();
 
-		$this->plugin_main = new Plugin_Main( basename( dirname( __DIR__ ) ) . '/plugin-check.php' );
+		$this->plugin_main = new Plugin_Main( WP_PLUGIN_CHECK_MAIN_FILE );
 	}
 
 	public function test_context() {