The Doctor and Sarah have landed on the Siccati colony of Cerulean. They meet sculptor Pug>llism and admire his creation. Suddenly the air raid alarms sound, and Pug insists they go to the Hippodrome, the only safe place. The Siccati head for the Hippodrome, but several are still left outside when the attack comes. From inside the Hippodrome, the Doctor, Sarah and Pug watch as the force field opens, and Pug's creation fires at the attacking rocket from Vermill. It is ineffective, however, and the rocket fires on the colony. Hundreds of Siccati are killed.
The Hippodrome rocks with the blast, and Pug explains to the visitors that the colony is actually a spaceship that rides on the ice flows of the planet. The rocket has damaged the stabilisers. The Doctor helps to repair the stabilisers and upgrade Pug's defensive weapon. Meanwhile, Sarah makes friends with Sigma<Tism, a poet and the wife of the colony's leader, Tene>Brism. Sigma lets Sarah examine the scribe-tablet she uses to compose her work.
When the next attack comes, the Doctor herds Sarah and the surviving Siccati into the TARDIS. The new weapon successfully destroys the attacking rocket, but the Doctor has discovered that Vermill is actually Sedna, a planet in the solar system that hasn't been discovered by Sarah's time. Cerulean is in fact the planet Neptune.
With the destruction of the Vermill rocket, the Vermillons want to discuss peace with the Siccati. The Vermillons are sending strangers on their world to be their intermediaries, and the Siccati ask the Doctor to be their intermediary. However, he has detected a time ram and realises that he and Sarah must leave. Both Sarah and the Siccati are upset that he will not stay to help ensure peace.
As the Doctor and Sarah leave, he explains to her that the other TARDIS is an echo of his, and Sarah happily realises that the Doctor is coming back to help.
On Neptune, Sigma and Pug watch in despair as the TARDIS leaves. Seconds later, it reappears. This time a young man steps out, holding Sigma's scribe-tablet, and says, "Is this yours?"
- Jeremy Fitzoliver makes a cameo at the end of the story, in a scene set after Sedna, the final story of this anthology. In the audio release he is replaced by Harry Sullivan.
- A reading of the story by Beth Chalmers was released to subscribers whose subscriptions included The Doomsday Quatrain or House of Blue Fire.