When a giant great white shark terrorizes a New England beach town, a local sheriff must team up with a marine biologist and a grizzled fisherman to hunt down and destroy the beast before it can claim any more lives.

I suppose, in simple terms, the aim of a logline is to summarise a compelling and developing conflict, it's genre and key characters.

Loglines for action-adventure stories usually feature an active protagonist, but when it comes to thrillers the hero often starts out as a victim or in the case of neo-noir as an antihero. In both cases these characters are reacting to the antagonist who drives the plot. In these cases, the logline gets a bit tricky.