The time assassins were a group of people who ripped their targets out of history by destroying their ancestors.
They were mutated into squid-like beings upon using an experimental salvaged time machine on 27th century Mars, giving themselves an effective "death warrant" which scrambled their genetic code. They intended to wipe out the entire lineage of President Lithops by killing the Mayor of London in the 21st century. The Tenth Doctor, who was unaffected by the machine, followed the assassins to the 21st century in the machine, but more assassins showed up at the laboratory forcing the Doctor's companion Martha Jones to retreat from the lab. In the 21st century, the Doctor saved the Mayor by creating a decoy "soup" of the Mayor's DNA, causing the assassins to follow the Doctor and his flask of soup on a London bus. DI Moloch helped attack the assassin controlled lab, and Martha brought the Doctor back to the 27th century when she reversed the controls of the time machine. To prevent anyone else from being tempted to fiddle, the Doctor destroyed the machine. The destruction of the machine also caused the time assassins in London to disappear when the connection was terminated. (COMIC: Bus Stop!)