A dozen passengers are trapped in the subway with a killer on the loose. Inside the cars and in the surrounding darkness of the tracks, characters are mysteriously picked off by a cunning killer; alliances form and finger pointing ensues, as each believes another is guilty. The whodunit unfolds in the manner of a classic Agatha Christie mystery, but with the grit and pulp of modern-day New York City.

On a non-interactive platform like a theater or a television, 3RD RAIL plays like a standard film. It automatically switches between the different storylines of each subway car in a behavior known as the Director's Cut, which allows 3RD RAIL to be watched linearly and exhibited as a feature film.

However, 3RD RAIL's primary interactive element – and what sets it apart from traditional films – is the ability to deviate from the Director's Cut and choose which subway car to view in real-time. By using an input device on any connected platform – a mouse, remote control, touch screen, or keypad – viewers can instantly switch between multiple simultaneous storylines and uncover clues on their own.

Clues, interactions, and even killings will be "hidden" outside of the Director's Cut, while viewer-triggered flashbacks incrementally reveal the backstory and form another layer of mystery to unravel. Through triggered flashbacks and simultaneous, real-time storylines, 3RD RAIL's wealth of hidden content creates meaningful rewards and implores viewers to replay each episode in an attempt to solve the mystery.