diff --git a/README.md b/README.md
index 597ad59..ed41a6c 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,9 @@
 # s2watchdog
 
-A simple watchdog script for the deep freezer alert system in our s2 scullery.
\ No newline at end of file
+A simple watchdog script for the deep freezer alert system in our s2 scullery.
+
+The alert system of our deep freezer in the s2 scullery will send emails as 
+soon, as the temperature exceeds a certain level. 
+
+This script will check regularly if the alert system itself is reachable and
+send an email if the system is not available (network down or power failure).
\ No newline at end of file
diff --git a/s2watchdog/__init__.py b/s2watchdog/__init__.py
index 966eb36..f6219c7 100644
--- a/s2watchdog/__init__.py
+++ b/s2watchdog/__init__.py
@@ -1,2 +1,128 @@
-def test():
-    print('TESTRUN')
+''' Simple watchdog script for the deep freezer alert system
+
+The alert system of our deep freezer in the s2 scullery will send emails as
+soon, as the temperature exceeds a certain level.
+
+This script will check regularly if the alert system itself is reachable and
+send an email if the system is not available (network down or power failure).
+'''
+
+import json
+import requests
+
+from datetime import datetime
+
+from . import pysema
+
+
+
+# alert emails recipients, subject and message
+EMAIL_RECIPIENTS = ['frey@imtek.de']
+EMAIL_SUBJECT = '[S2 Scullery]: Deep Freezer Alert System Is Offline'
+EMAIL_BODY = [
+    'The -80 alert system in the S2 scullery seems to be offline.',
+    'Maybe the network is down, there is a power outage or the system crashed.',
+    '',
+    '>>> Please have a look as soon as possible. <<<',
+    '',
+    'The system was last seen on {} (UTC)'
+    ]
+
+# which url to check
+ALERT_SYSTEM_URL = 'http://test.cpi.imtek.uni-freiburg.de'
+
+
+# How many times can an error be ignored?
+GRACE_PERIOD = 4
+
+# where to store the data and default values
+DATA_STORE_PATH = '/val/local/s2watchdog.json'
+DEFAULTS = {
+    'emails_sent': 0,
+    'last_seen': 'Never',
+    'last_result': '',
+    'time_to_live': 0,
+    'url': ALERT_SYSTEM_URL,
+    }
+
+# how long (in seconds) to wait for a response from the alert system
+TIMEOUT = 5
+
+
+
+def load_data(path):
+    ''' load and decode the saved data '''
+    try:
+        with open(path, 'r') as file_handle:
+            data = json.load(file_handle)
+
+        # ensure, that these two keys are integers:
+        for key in ['time_to_live', 'emails_sent']:
+            value = data.get(key, None)
+            if not isinstance(value, int):
+                data[key] = 0
+        return data
+    except (FileNotFoundError, AttributeError):
+        return DEFAULTS
+
+
+def save_data(path, data=None)
+    data = data or DEFAULTS
+    with open(path, 'w') as file_handle:
+        json.dump(data, file_handle)
+
+
+def query_alert_system(url):
+    response = requests.get(
+        url,
+        allow_redirects=False,
+        timeout=TIMEOUT
+        )
+    response.raise_for_status()
+    return response.text
+
+
+def send_alert_mail(data):
+    headers['Reply-To'] = EMAIL_RECIPIENTS
+    body = '\n'.join(EMAIL_BODY)
+    pysema.send(
+        EMAIL_RECIPIENTS,
+        EMAIL_SUBJECT,
+        body.format(data['last_seen']),
+        headers
+        )
+
+def run():
+    data = load_data(DATA_STORE_PATH)
+
+    # the url has changed, use the default values
+    if data['url'] != ALERT_SYSTEM_URL:
+        data = DEFAULTS
+
+    try:
+        result = query_alert_system(ALERT_SYSTEM_URL)
+        if result != data['last_result']:
+            # the retrieved result was not cached somehow
+            # everything is all right!
+            # we'd return after the data is saved
+            data = {
+                'emails_sent': 0,
+                'last_seen': datetime.utcnow().isoformat(),
+                'last_result': result,
+                'time_to_live': GRACE_PERIOD,
+                'url': ALERT_SYSTEM_URL,
+                }
+            save_data(DATA_STORE_PATH, data)
+            return
+    except requests.exceptions.RequestException:
+        # the alarm system could not be reached.
+        # but maybe its in the grace period, so we pass here
+        pass
+
+    ttl = data['time_to_live'] - 1
+    if ttl <= 0:
+        data['email_sent'] = data['email_sent'] + 1
+        data['time_to_live'] = GRACE_PERIOD * data['email_sent']
+        save_data(DATA_STORE_PATH, data)
+        send_alert_mail(data)
+
diff --git a/s2watchdog/pysema.py b/s2watchdog/pysema.py
index 4610b13..11b1099 100644
--- a/s2watchdog/pysema.py
+++ b/s2watchdog/pysema.py
@@ -21,12 +21,12 @@ class SendMailException(Exception):
     pass
 
 
-def send(to=None, subject='', message='', **headers):
-    msg = create_message(to, subject, message, **headers)
+def send(to=None, subject='', message='', headers=None):
+    msg = create_message(to, subject, message, headers)
     send_message(msg)
 
 
-def create_message(to=None, subject='', message='', **headers):
+def create_message(to=None, subject='', message='', headers=None):
     if isinstance(to, str):
         recipients = to
     elif isinstance(to, (list, set, tuple)):
@@ -34,6 +34,9 @@ def create_message(to=None, subject='', message='', **headers):
     else:
         raise SendMailException('No recipients given')
 
+    if not isinstance(headers, dict):
+        headers = {}
+
     # ensure that the 'from' keyword (if given) is uppercase
     sender_mail = headers.pop('from', None)
     if sender_mail is not None:
diff --git a/setup.py b/setup.py
index 4117d12..def3557 100644
--- a/setup.py
+++ b/setup.py
@@ -7,7 +7,8 @@ with open(os.path.join(here, 'README.md')) as f:
     README = f.read()
 
 requires = [
-]
+    'requests'
+    ]
 
 setup(
     name='s2watchdog',
@@ -29,7 +30,7 @@ setup(
     install_requires=requires,
     entry_points={
         'console_scripts': [
-            's2watchdog = s2watchdog:test',
-        ],
-    },
-)
+            's2watchdog = s2watchdog:run',
+            ],
+        },
+    )