diff --git a/config/paths.js b/config/paths.js
index fbbeefc..85213cc 100644
--- a/config/paths.js
+++ b/config/paths.js
@@ -61,6 +61,7 @@ module.exports = {
     "commands/combine": resolveApp("src/cli/commands/combine.js"),
     "commands/graph": resolveApp("src/cli/commands/graph.js"),
     "commands/plugin-graph": resolveApp("src/cli/commands/pluginGraph.js"),
+    "commands/start": resolveApp("src/cli/commands/start.js"),
     sourcecred: resolveApp("src/cli/sourcecred.js"),
     fetchAndPrintGithubRepo: resolveApp(
       "src/plugins/github/bin/fetchAndPrintGithubRepo.js"
diff --git a/src/cli/commands/start.js b/src/cli/commands/start.js
new file mode 100644
index 0000000..62cba75
--- /dev/null
+++ b/src/cli/commands/start.js
@@ -0,0 +1,117 @@
+// @flow
+
+import {Command} from "@oclif/command";
+import chalk from "chalk";
+import child_process from "child_process";
+import express from "express";
+import {choosePort} from "react-dev-utils/WebpackDevServerUtils";
+import tmp from "tmp";
+
+import {sourcecredDirectoryFlag} from "../common";
+
+// Makes the script crash on unhandled rejections instead of silently
+// ignoring them. In the future, promise rejections that are not handled will
+// terminate the Node.js process with a non-zero exit code.
+process.on("unhandledRejection", (err) => {
+  throw err;
+});
+
+const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 4000;
+const HOST = process.env.HOST || "0.0.0.0";
+
+export default class StartCommand extends Command {
+  static description = "start a web server to explore the contribution graph";
+
+  static flags = {
+    "sourcecred-directory": sourcecredDirectoryFlag(),
+  };
+
+  async run() {
+    const {flags: {"sourcecred-directory": sourcecredDirectory}} = this.parse(
+      StartCommand
+    );
+    startServer(sourcecredDirectory);
+  }
+}
+
+async function startServer(sourcecredDirectory: string) {
+  let server, webpack;
+  function cleanup() {
+    if (server && server.listening) {
+      server.close();
+    }
+    if (webpack) {
+      webpack.kill();
+    }
+  }
+
+  let shuttingDown = false;
+  ["SIGINT", "SIGTERM"].forEach((signal) => {
+    process.on(signal, () => {
+      if (shuttingDown) {
+        // Force shut-down.
+        process.exit(2);
+      } else {
+        shuttingDown = true;
+        console.log("\nShutting down.");
+        cleanup();
+      }
+    });
+  });
+
+  const webpackWorkdir = tmp.dirSync({unsafeCleanup: true}).name;
+
+  console.log(chalk.bold("Starting Express..."));
+  const expressApp = createExpressApp(webpackWorkdir, sourcecredDirectory);
+  server = await new Promise(async (resolve, _unused_reject) => {
+    const port = await choosePort(HOST, DEFAULT_PORT);
+    let server = expressApp.listen(port, () => {
+      resolve(server);
+    });
+  });
+  server.on("close", () => {
+    console.log(chalk.bold("Express server closed."));
+    cleanup();
+  });
+  console.log(
+    chalk.green(`Server listening on port ${server.address().port}. `) +
+      `You might want to wait for Webpack to say ${chalk.bold("[built]")}.`
+  );
+  console.log();
+
+  console.log(chalk.bold("Starting Webpack..."));
+  webpack = startWebpack(webpackWorkdir);
+  webpack.on("exit", (code, signal) => {
+    console.log(
+      `${chalk.bold("Webpack exited")} with ${code} (signal: ${signal})`
+    );
+    cleanup();
+  });
+}
+
+function createExpressApp(webpackWorkdir, sourcecredDirectory) {
+  const app = express();
+  app.use(express.static(webpackWorkdir));
+  app.use("/__data__", express.static(sourcecredDirectory));
+  return app;
+}
+
+function startWebpack(workdir: string) {
+  const webpack = child_process.spawn(
+    "yarn",
+    [
+      "--silent",
+      "webpack",
+      "--config",
+      "./config/webpack.config.dev.js",
+      "--output-path",
+      workdir,
+      "--watch",
+    ],
+    {
+      env: {...process.env, NODE_ENV: "development"},
+      stdio: "inherit",
+    }
+  );
+  return webpack;
+}