The title of the story indicates that Mrs. Foster's way to heaven on earth is to kill her husband (send him to heaven) by leaving him trapped between floors going upstairs in their New York home's elevator.

The elderly Mr. Foster has for years sadistically tormented his equally elderly wife. She experiences anxiety at the idea of being late for events. Knowing this, he always manages to arrange affairs so that she is running late. He delays her in passive aggressive ways, and she is afraid to challenge him.

We see how he enjoys tormenting her about possibly missing a flight to Paris, where she is going to spend six weeks visiting her grandchildren. When the flight is delayed due to fog, she wishes to spend a night in a hotel, but he insists she come home. The next morning, he does everything he can to ensure she is late once again.

When she returns to the house briefly before the trip, she realizes he is trapped between floors on their elevator. Instead of saving him, she goes on her journey and leaves him in the elevator to die. Six weeks later she returns home.

We note that she repays his passive-aggressive behavior with a passive-aggressive response to his being stuck in the elevator. By all indications, she will be much happier—in heaven, so to speak—now that he is gone.

The reader will not understand the title of the story until he reaches the end. The title is intentionally ambiguous. It suggests both that Mr. Foster was riding up towards heaven when he went back into the townhouse on the false pretext of looking for a little package, and also that his demise was going to be a blessing for his wife because he had made her life just the opposite of heaven.

They own one of those narrow, six-story townhouses in Manhattan. It is so tall, and they are so old, that they have to use the elevator to get up and down. Mrs. Foster found out, as she had suspected for some time, that her husband was deliberately playing sadistic games with her, making her late to important engagements just because he knew this was her weak spot. Roald Dahl opens the story with this essential characterization:

All her life, Mrs. Foster had had an almost pathological fear of missing a train, a plane, a boat, or even a theatre curtain. In other respects, she was not a particularly nervous woman, but the mere thought of being late on occasions like these would throw her into such a state of nerves that she would begin to twitch. 

The title is telling the reader, without the reader's knowing it, that Mr. Foster was dead and in heaven, giving the old man the benefit of the doubt. Also that Mrs. Foster is now in heaven on earth because she has everything she could possibly want, including her freedom from a sadist, a luxurious townhouse, millions in financial assets, and the ability to visit her daughter in Paris whenever she feels like it. The words "The Way" in the title suggest that the way she transformed her unhappy life was by yielding to a sudden impulse to kill her husband through the same kind of "passive aggression" that he had always practiced on her. Listening at the front door, she heard sounds that told her the elevator was going to get stuck between floors. Since they had closed the house up for six weeks, all she had to do was to do nothing. If she didn't go back inside, her husband would be stuck in the elevator for six weeks and would starve to death before it was time for her to return from Europe.

She may have technically been guilty of murder or negligent homicide, but it would have to be proved that she knew her husband was stuck in an elevator. And there would be no way to prove that because she was the only one in the whole wide world who could know what she knew. Even the author Roald Dahl does not state in so many words that this foxy lady knew what she knew. At the end he only has her call the elevator maintenance service and have a one-way conversation over the phone.

“Hello,” she said. “Listen - this is Nine East Sixty-second Street . . . . Yes, that’s right. Could you send someone round as soon as possible, do you think? Yes, it seems to be stuck between the second and third floors. At least, that’s where the indicator’s pointing . . . . Right away? Oh, that’s very kind of you. You see, my legs aren’t any too good for walking up a lot of stairs. Thank you so much. Good-bye.”

She replaced the receiver and sat there at her husband’s desk, patiently waiting for the man who would be coming soon to repair the lift. 

We can well imagine the sight the repairman is going to see when he gets there. But that sight is left entirely to our imaginations. Mrs. Foster won't even see the body because she establishes that her legs are not too good for walking up a lot of stairs.