Ep 3. Gridlock

In the third season of Doctor Who, episode 4, "Gridlock," the Doctor and his companion Martha find themselves trapped in an endless traffic jam on the planet New Earth. The road they are on, the Motorway, stretches for miles and is lined with dozens of different species of aliens who have come to New Earth for various reasons. The Doctor and Martha start to investigate why they are stuck in traffic, and quickly realize that something sinister is going on.

As they try to make their way through the sea of cars, the Doctor and Martha encounter a variety of New Earth's inhabitants, including a cat-like humanoid named Thomas Kincade Brannigan and his pregnant wife Valerie. The couple are on their way to a "haven" at the end of the Motorway, where they hope to start a new life away from the pollution and danger of the city.

Meanwhile, Martha becomes separated from the Doctor and ends up on a car full of strangers, each with their own story to tell. She meets a young woman named Cheen who is searching for her lost sister, as well as several elderly passengers who have been on the Motorway for years, waiting for it to clear up.

As the Doctor continues to investigate, he discovers that the traffic jam is not a natural phenomenon, but is actually being caused by a group of alien creatures known as the Macra. The Macra have taken control of the city and are using the traffic as a way to feed on the exhaust fumes of the cars, which is the only source of sustenance on the planet.

Determined to save the city, the Doctor enlists the help of the other stranded passengers and leads them on a mission to take down the Macra and free the city from their control. Along the way, they encounter countless obstacles and challenges, including terrifying creatures and unexpected surprises.

In the end, the Doctor and his friends are able to overcome the Macra and bring down the barriers that have been keeping them trapped on the Motorway. They emerge into the sunlight, free at last to explore the city and start new lives. As they say their goodbyes and part ways, the Doctor and Martha continue on their journey, eager to see where their travels will take them next.