Rather than comment on the virtues of what many consider to be one of the best films ever made, I'm going to do my best to dumb down, and in many ways, destroy the beautifully written plot by reinterpreting it in the worst way possible.
It's a hot summer in Manhattan. The perfect storm for Jeff the pervert photographer who is handicapped with broken leg. All he can do is stare out his window at the microcosm his courtyard presents. Lucky for him, neighbors' windows are wide open in an attempt to keep cool and he can see e-v-e-r-y-t-h-i-n-g. High on pain killers and hopped up on Coca-Cola, he puts his eye to the viewfinder of his camera to get a closer look (spy) at what his neighbors are doing from the confines of their apartments. A space that was intended to private. Those rules however do not apply to Jeff. Over the course of his recovery, he sees and hears things which, although not abnormal, his hallucinating brain interprets as murder.
A man casually cleaning a knife and handsaw---He obviously must have used it to kill his wife and chop her up so she would fit in an easy-to-dispose suitcase.
A dead dog---There's no way that dog died of old age or a street disease. Someone must have broken its neck because apparently they couldn't get their hands on some poison. Maybe even the same man who hacked up his wife. He must have feared the dog would sniff out his wife's dismembered corpse and start digging for bones, logically.
As the plot progresses, Jeff is still a pervert but we find out his accusations aren't as wild and unfounded as we first thought. What we thought were painkillers were actually peppermint tic-tacs. What we thought was Coca-Cola, was actually Pepsi. Knowledge of the truth is just too much for Jeff to handle like a reasonable human-being. Naturally, he processes this heavy reality by acting like a complete moron and plays mind games with a suspected murderer---because that is what you do when you think someone is guilty of murder.
With time, the alleged killer puts the two pieces of Jeff's puzzle for dummies together. His eyes turn towards Jeff with an intent to kill. He even gives a courtesy call to advise of his visit. With little mobility and only camera flashes for self-defense, Jeff attempts to blind the alleged killer as he breaks through the front door à la Kool-Aid man. Surprisingly, Jeff's feeble attempt to save his own life is successful for a few seconds. After regaining visibility, the killer channels his inner Hulk and pushes Jeff out the same window he used to fulfill his pervy desires.
In the end, the killer is caught and Jeff gets himself another broken leg.
If you've ever wondered what happens when an injured Peeping Tom photographer gets their hands on an Exakta VX equipped with a 400mm lens, look no further than Rear Window.