HMS Vigilant was a Royal Navy nuclear submarine sent to investigate strange weather patterns that had made an oil rig disappear and downed a helicopter in the North Sea. Its registry number was 517. The Fourth Doctor tried to gain access to it. He wanted to stop it because he sensed it was sailing into an alien trap. However, he was denied permission by Admiral Slingsby. Soon after launch, the ship was ripped in half and its entire crew died. (COMIC: The Sinister Sea)