The Saint Simon Templar (Louis Hayward) returns to England after a call for help from his socialite friend, before he can arrive she has died in a "car accident" (assisted by being chased by hoodlums).
The Saint knew his friend had a gambling habit and was probably in debt, he begins his investigation then in the underground gambling scene (gambling being illegal in the UK at the time) in 1950s London. His investigation leads him to the brutal River Gang who seem to be behind his friend's death and much more besides...
This is a decent film, rather grubby and hard for a British film of the period. It also is an interesting view of England still recovering from the war. The plot can be a bit slow at times though. See Diana Dors in an early role.