A British Major and aristocrat must team up with a vulgar guerilla leader in order to blow up a bridge in Nazi occupied Greece. Based on a true story.

Maybe something like:

The true story of an aristocratic British major and a vulgar guerrilla leader who team up to choke off supplies to Rommel’s army in North Africa by destroying strategic railroad bridges in Nazi occupied Greece.
(35 words)

I tweaked it a little because given how mountainous the landscape of Greece is, I imagine that there were many bridges that would have been targets for sabotage.? If, however, they teamed up only to target the most important one, the one that would be hardest for the Nazis to rebuild, then amend accordingly.

fwiw

Based upon a true story, an aristocratic British major must team up with a vulgar guerilla leader to blow up a key bridge in Nazi occupied Greece.”
(27 words)

I’m assuming that “British major and aristocrat” are descriptive terms for the same person.

I would lead off with “Based upon a true story”.? And I would append a ticking clock to heighten the sense of dramatic urgency and import.? They must blow up the bridge before… before what?