The HMS Thunder was a Royal Navy submarine. In 1941, during World War II, The Seventh Doctor and Ace helped Captain Thomas Fitzgerald and the crew of the Thunder to rescue survivors from the sunken HMS Intrepid Ranger and dispatch a preying U-boat. However, the Thunder was hit by a torpedo and began to sink. The Doctor and Ace departed to prevent changes to history as the endangered crew evacuated. (AUDIO: Dark Convoy)