Nobita is in trouble when his mother finds a test paper he failed in, in his drawer. Nobita wants a lock so Doraemon gives him a key which can lock anything. Once done, only the user can open or shut it, using the words: Open Sesame or Close Sesame. The episode ends up with Nobita locked out of the house in heavy rain because he has a cold which has changed his voice so the key no longer recognizes him.
