Strepto was the home planet of the Water Hags. (PROSE: Something in the Water) Like the other stolen planets taken by the Daleks to build their reality bomb, (TV: Journey's End) Strepto seemed to inexplicably vanish. Because she was travelling off-world during the planet’s disappearance, Sally Blackteeth became Strepto's sole survivor, eventually fleeing through the Rift to Earth during the Middle Ages. (PROSE: Something in the Water) All 27 stolen planets were returned to their proper locations by the Doctor's TARDIS. (TV: The Stolen Earth)
Behind the scenes
Something in the Water insinuates that Strepto is one of the 27 planets stolen by the Daleks in Doctor Who series 4; this was later echoed in the non-narrative source Doctor Who: The Time Traveller's Almanac.