In an attempt to lift the Doctor out of his irritable and erratic mood, Bernice suggests he investigates the mystery of the Seven Planets — an entire planetary system that disappeared without trace several decades before Bernice was born. One of the Seven Planets is a nameless giant, quarantined against all intruders. But when the TARDIS materialises, it becomes clear that the planet has other visitors: a hit-squad of killer androids; a trespassing scientist and his wife; and two shape-changing criminals with their team of slaves.