Philip Weaver is having an affair with his boss's wife, and he's getting tired of seeing her irregularly. He arranges for Mr. Barrett to travel on an urgent business trip, and his intent is that Mr. Barrett never arrive at his final destination. As part of the plan, he gives Mrs. Barrett explicit written instructions on her role: she is to write a letter to her husband saying how much she misses him, invite several friends over for a drink the day he travels, and, whatever else she might do, destroy the instructions. Being very dumb, she realizes too late that she must have included the instructions in the letter to her husband. She frantically contacts the post office to see if she can retrieve the incriminating correspondence and is relieved when the letter is returned to the apartment doorman for insufficient postage. Unfortunately, the doorman is very efficient.
—garykmcd