In 1955, a young Debutante is sole witness to murder and must enlist the help of another criminal, his niece, to help her get to safety. But the killer is on their trail and he?ll stop at nothing to silence them.

The niece of the killer, sorry I worded it all wrong.

I realise I've worded it incorrectly. It's meant to read...

In 1955, a young Debutante is sole witness to murder orchestrated by London's most ruthless crime boss. She must enlist the help of another criminal, his niece, to help her get to safety. But the killer is on their trail and he?ll stop at nothing to silence them.

But that sounds too long winded. It was originally:

A young Debutante must enlist the help of a criminal Teddy girl to get her to safety after she is sole witness to a murder orchestrated by London's most ruthless crime boss.

But this is a bit lack luster.

The 1955 part is essential to the story. It's about a Teddy Girl and a Debutante and Teddy Girls/Boys were only part of the 1950s/60s and Debutantes aren't really Debutantes in England anymore.

Agree with sharkeatingman - unless the time period is essential to your script, leave it out or move your story to present day. My guess is the murder victim is an historical character tied to that time period, maybe? I was a little confused by the wording - the criminal who the debutante enlists is the murder victim's niece? So, two chicks on the run? Could be an interesting play on a Thelma and Louise theme; haven't had one of those since...well, since Thelma and Louise. Still missing a hook though.
Protag - debutante
Antag - killer
Goal - try to not get killed/get to safety
Hook - What's the hook? Is the killer someone she knows and trusts?

There has to be something unique about your story that sets it apart from the very familiar story that you're portraying in your logline.