diff --git a/src/index.js b/src/index.js
index aa4b1ed..69f7fdf 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,10 +1,81 @@
 'use strict'
 
+const { pathToFileURL } = require('node:url')
+const fs = require('node:fs')
+
+/**
+ * Calls the provided function for each matching token.
+ *
+ * @param {Object} params RuleParams instance.
+ * @param {string} type Token type identifier.
+ * @param {Function} handler Callback function.
+ * @returns {void}
+ */
+const filterTokens = (params, type, handler) => {
+  for (const token of params.tokens) {
+    if (token.type === type) {
+      handler(token)
+    }
+  }
+}
+
+/**
+ * Adds a generic error object via the onError callback.
+ *
+ * @param {Object} onError RuleOnError instance.
+ * @param {number} lineNumber Line number.
+ * @param {string} [detail] Error details.
+ * @param {string} [context] Error context.
+ * @param {number[]} [range] Column and length of error.
+ * @param {Object} [fixInfo] RuleOnErrorFixInfo instance.
+ * @returns {void}
+ */
+const addError = (onError, lineNumber, detail, context, range, fixInfo) => {
+  onError({
+    lineNumber,
+    detail,
+    context,
+    range,
+    fixInfo
+  })
+}
+
+const EXTERNAL_PROTOCOLS = new Set([
+  'http:',
+  'https:',
+  'mailto:',
+  'tel:',
+  'ftp:'
+])
+
 const customRule = {
   names: ['relative-links'],
   description: 'Relative links should be valid',
   tags: ['links'],
-  function: () => {}
+  function: (params, onError) => {
+    filterTokens(params, 'inline', (token) => {
+      token.children.forEach((child) => {
+        const { lineNumber, type, attrs } = child
+        if (type === 'link_open') {
+          attrs.forEach((attr) => {
+            if (attr[0] === 'href') {
+              const href = attr[1]
+              const url = new URL(href, pathToFileURL(params.name))
+              url.hash = ''
+              const isRelative =
+                href.startsWith('./') ||
+                href.startsWith('../') ||
+                !EXTERNAL_PROTOCOLS.has(url.protocol)
+              if (isRelative && !fs.existsSync(url.pathname)) {
+                const detail = `Link "${href}" is dead`
+                addError(onError, lineNumber, detail)
+              }
+            }
+          })
+        }
+      })
+    })
+  }
 }
 
 module.exports = customRule