What is the epiphany in "Greasy Lake"?

One night after he runs away from the scene of a vicious fight, the protagonist decides to hide at Greasy Lake. He's afraid that the police will arrive at the scene and arrest him if they find him.

While he's at the lake waiting for an opportune time to leave, he finds a dead man floating in the water and starts to think about all the things that could have happened to get the man to that point. The dead man was probably someone just like him—someone who acted big and tough and cool but ended up getting murdered during a fight.

Then the protagonist sees that this is exactly the sort of thing that could happen to him at any time. That is his epiphany—that if he keeps on making sex, drugs, and alcohol the most important things in his life, he will end up dead, just like the man in the lake.

What is the epiphany in "Greasy Lake"?

The unnamed narrator of "Greasy Lake" arrives at the lake with his friends at night to party. He thinks he is tough. He and his friends end up with more trouble than they bargained for, including a fight in which the narrator thinks he's killed his opponent (he hasn't). After the friends of the downed man chase them away, at which point the narrator drops his keys in the grass at the wrong moment so that he can't drive off, he eventually witnesses his opponents smashing up his mother's car. 

When it is all over and dawn is breaking, the narrator's epiphany is that he is not as tough as thought he was. He is sobered by his experience and begins to mature as he recognizes that what he considered fun and games really could have had far more serious consequences.

What is the epiphany at the end of Greasy Lake?

The character at the beginning sees the the nature as antagonistic and a one-lane shopping mall, somewhere boring and uneventful. The protagonist and his friends were acting as if 'courtesy and winning ways went out of style' and behaved recklessly although most of the pranks were harmless. They decided to pull a prank on their friend 'Tony Lovett' and found a car on the top the hills, only to find that his friend was in fact a 'greasy' big man. They are soon involved in a violent fight. The protagonist strikes the guy with a tire iron and the big guy falls to the ground unconscious, then runs to the island, away from the crime scene. When he trips over and falls into the lake to find the dead body of 'Al', he experiences an awakening. The falling in the lake represents his rebirth and a spiritual reassessment where his view on nature changes completely. The 'big man' coming back to life represents a prize or award of the character's rebirth and he describes the nature as 'primordial' which directly contrasts his previous views on nature. Even when the girls ask them to join them later on, he refuses the offer, proving how influential and effective his epiphany after the incident was.