The season revolves around Veronica Mars, a high school student and private investigator in the fictional Southern California seaside town of Neptune. When Veronica's best friend, Lilly Kane, is murdered, her life falls apart. Her father, County Sheriff Keith Mars, accuses Lilly's father of being involved in the murder. This provokes Neptune's wrath and Keith's ousting as sheriff in a recall election. Veronica's mother, Lianne, develops a drinking problem and leaves town. Veronica's "09er" friends—wealthy students from the fictional 90909 ZIP code—force her to choose between them and her father; she chooses her father. Keith opens a private investigation agency, Mars Investigations, where Veronica works part-time.

The season begins with the introduction of two new cases: when a school bus full of Neptune High students plunges off a cliff, killing almost everyone on board, Veronica Mars makes it her mission to discover why the bus crashed. Logan Echolls is accused of killing a Pacific Coast Highway biker gang member after drunkenly picking a fight with Eli "Weevil" Navarro and the PCHers.

The season continues the story of Veronica Mars, now a freshman studying at Hearst College while moonlighting as a private investigator under the wing of her detective father.

Mystery. Parties. Scandal. Corruption. Season 4 is going be the most explosive one yet. Spring breakers are getting murdered in Neptune. After Mars Investigations is hired by the family of one of the victims to find their son’s killer, Veronica is drawn into an epic mystery that pits the enclave’s wealthy elites, who would rather put an end to the month-long bacchanalia, against a working class that relies on the cash influx that comes with being the West Coast’s answer to Daytona Beach.