Motive V. Oppertunity

In this short story written by Agatha Chrisite, there is a sealed envelop containing a legal and signed will. However, when it is necessary for the will to be seen after being locked up in a safe for the past two months, the paper within the envelope, the will, is blank. Mr. Petherick, the charecter telling the story, recalls the scene in which the will was written and sealed, pointing out that only five people had previously handled the envelope. Two of the people whom had touched the will both had opportunity to tamper with it, however doing so would not benefit either because the will was in their favor. The other three people whom had handled it had motive but no opportunity. The story ends with Miss Marple, the short stories main character, stating that the pen used to write the will had disappearing ink. The maid whom had handed the writer of the will the pen did not agree with the dividing of the property and money, and therefore gave a pen containing disappearing ink so that those dividing would not happen.