A logline is a one-sentence description of your story concept.
The purpose of a logline is to present the essence of your screenplay in a clear, entertaining way that hooks the reader and makes him/her want to read more. When constructing a logline, it is helpful to keep the following points in mind:
1. The logline really should be only one sentence long. Many writers – some who find it hard to boil their ideas down into a single line and others who perhaps think that the one sentence notion is just a loose guideline -- stretch them to two, three, and sometimes even more lines. The problem is that a multi-sentence logline is not a logline; it is a synopsis. While the point of a synopsis is to convey the structure and key elements of a story, a logline is simply meant to communicate the overall premise of your piece, so don't weigh it down with lots of plot and/or character details.
Here is an example for a poorly crafted logline for The Godfather:
Don Vito Corleone is a powerful Mafia head with three sons. The youngest son, Michael, a decorated World War II veteran, wants nothing to do with the family business, but when Vito is wounded in an assassination attempt, Michael comes to his rescue and guns down the men responsible for the attack on his father. After spending some time in Sicily, where he learns the history of the Mafia and suffers a terrible personal loss when his wife is killed, Michael returns home, takes his father's place, and settles all scores by wiping out all of the family's enemies on a single day.
Here is a much better example:
The idealistic son of a Mafia Don is drawn into the mob and ultimately assumes his father's role as the head of the family's nefarious business.
2. The wording of your logline should be simple, direct, and to the point -- whenever possible, avoid elaborate sentence structure and flowery verbiage.
Here is an example of an excessively worded logline for Jaws:
When a giant great white shark begins attacking and killing swimmers in the waters off of a small New England summer resort community just before the all-important Fourth of July weekend, the town's police chief joins forces with a scientist and a grizzled old fisherman with a hatred of sharks and the trio set sail in a rickety old boat to hunt and kill the monster.
Here is a simpler, more effective alternative:
A cop, a scientist, and a fisherman attempt to kill the giant great white shark that has been attacking bathers in the waters off of a small summer resort town.
3. Your logline needs to clearly express your specific concept, so don't be too generic ("A giant shark kills people in Massachusetts") or too opaque ("When the town fathers of Amity put commerce ahead of safety, toothy retribution soon arrives in the form of a pair of prehistoric jaws").
4. A logline should describe your premise, not the formula behind the premise, so for Air Force One you should write something like: "The President of the United States fights back against the terrorists who hijack his plane and threaten his family" rather than "My script is a cross between The West Wing and Die Hard."
5. A logline should describe the story, not the theme, so for Gone with the Wind you should write something like "Scarlet O'Hara fights to save her family's land and legacy in the midst of the Civil War" rather than "As long as we stay connected to our roots, the human spirit can never be defeated."
by Ray Morton