Strike from the Sea (1978)

INDO-CHINA 1941 Cruising somewhere off Saigon is the world's largest and most dangerous submarine - the French Soufrière. A rich prize for the enemy, the British navy must capture her for themselves before she is used against them. For Commander Robert Ainslie, it represents the greatest challenge of his career. He must take the foreign submarine and use her against the enemy in the defence of Singapore...