The Doctor (Christopher Eccleston) takes Rose (Billie Piper) five billion years into the future. The TARDIS materialises on Platform One, a space station in orbit around Earth, where a grand party is celebrating the final destruction of the now-uninhabited planet as the sun expands. The Doctor uses his psychic paper as an invitation. He and Rose find an assortment of elite beings there, including Lady Cassandra (Zoe Wanamaker), the last human, reduced to a face on a stretched piece of skin, and the enormous Face of Boe, both of whom will re-appear later in the series. Someone refers to Rose as Bad Wolf, which becomes significant many episodes later.
The Adherents of the Repeated Meme bring gifts of robotic spiders who begin to take the station apart. The Steward of Platform One (Simon Day) knows something is amiss but is killed by the spiders when they expose him to the sun’s radiation. Rose insults Cassandra, and the Adherents knock her out and leave her in the observation room to be exposed in the same way. The Doctor stops the sun-filter from dropping but can’t free her immediately.
He knows the Adherents are responsible, but they are commanded by Cassandra. Her plan was to create a hostage situation with her as one of the victims and profit from the compensation, but now she will profit from the other guests’ deaths, as she has stock in all of their competitor’s companies. She teleports off the station, saving herself, as the destruction continues. The Doctor and a sentient tree (unrelated to Groot as far as I can tell) named Jabe (Yasmin Bannerman) travel to the centre of the station to remedy the situation, through several spinning fans. Jabe sacrifices herself and the Doctor succeeds just in time.
He reverses Cassandra’s teleportation and brings her back. The station is now hot and dry inside from the sun’s radiation and Cassandra desiccates and ruptures. Rose looks down at the debris of the Earth and laments that she is the last human being. The Doctor tells her that he is the last of his species too, as his home planet was destroyed in a great war.
Platform One was inspired by the Restaurant at the End of the Universe by Douglas Adams, a one-time writer for Doctor Who. The Face of Boe was inspired by the Guild Navigators in the 1984 film Dune. The Face of Boe would become a recurring image in the series for years. Lady Cassandra was inspired by women attending the Oscars—the most beautiful women in the world looking thin and sickly with weight-loss. The song Toxic by Britney Spears is played by Cassandra as a traditional Earth Ballad on her iPod—a jukebox. Cassandra has the Magna Carta and a copy of Harry Potter and the Philosopher’s Stone under glass. Zoe Wannamaker played Madam Hooch, the Quidditch teacher and referee in that series. Cassandra was singled out in the general praise for the episode as a triumph of special effects, of which there were 203 shots. The entire episode was conceived as a showcase for Doctor Who’s special effects and used up most of the season’s budget.