The TARDIS materialises inside a spacecraft. The Doctor and Sarah learn that they are in the Kuiper Belt, inside the hold of the spacecraft. An alarm sounds, and smoke pours through a door in the hold. They open the door and rescue an astronaut, Chang Hu. The small console room is on fire, and the Doctor puts it out, though there is much damage. Chang Hu is surprised by their appearance on the ship, but assumes they are from "the Corporation".
They pass through a greenhouse, where they find Lendal and Talbot. Chang Hu starts to tell the Doctor their story when Warry, another astronaut, buzzes on the intercom that he is trapped in the control room, on the far side of the science module. Another astronaut, Duncan, enters and informs Chang Hu that she cannot get to the science module. She thinks Barrett is in there. Warry notes that the instruments say the science module has been depressurised.
Apparently Barrett opened the airlock from the science module. The science module is littered with debris. At first Duncan suspects the Doctor and Sarah, but it doesn't take long to convince her and the others that they weren't involved. The Doctor has an idea but wants the others to figure it out.
The Doctor studies the airlock while Sarah helps the astronauts clean the science module. The Doctor reveals to Sarah that he thinks Barrett found something outside in the Kuiper Belt. Something that has been waiting since before the Solar System formed. He investigates the computer Barrett was using, and finds evidence of an encrypted data stream, hidden in the regular transmissions sent by the spacecraft back to Earth.
Chang Hu takes Sarah to the shower, and on the way he tells her his life story, how his father was part of the space program and intended for Chang Hu to be the first of the space men. After Sarah finishes her shower, she shows the Doctor what she discovered — there is a tiny camera in the shower. He tells her the astronauts are being monitored, but that it's not a big deal, and heads back to the computer.
Sarah talks with Talbot, mentioning the camera, but is surprised when Talbot doesn't know what she means. She brings the astronauts to the science module, where the Doctor shows them what he found. He plays the recording of Barrett's last actions. They all watch as Barrett types on the computer, stands up and looks right at the camera, shouting something, returns to the computer, then goes to the airlock and opens it.
The Doctor and Sarah learn that the astronauts don't know about the cameras that are monitoring them. Sarah, however, realises that Chang Hu knows about the cameras. He explains that their actions are being sent back to Earth as entertainment. This is done to raise funds for the space missions. The Doctor deduces that Barrett must have discovered the cameras, believed that the spacecraft was a fake, and walked outside the airlock to prove it. The other astronauts almost believe it's a fake as well, but Chang Hu convinces them they really are in space. They are angry at first, but he convinces them they will be famous when they return to Earth. The Doctor and Sarah decide they should leave while the cameras are still down.
Back in the TARDIS, Sarah is pleased that she was right this time.
- The Doctor eats jelly babies.
- Marsberries are grown on the spacecraft.
- The Doctor quotes Sherlock Holmes.
- The Doctor thinks giant telepathic bees are responsible for Barrett's death.
- The spacecraft launched in 2031 and took nine years to arrive in the Kuiper Belt.
- Chang Hu was born in 2009.
to be added
- The Doctor once again tells Sarah that it takes two minutes for radio signals to travel between Earth and Mars. (TV: Pyramids of Mars)