When their covers are blown on a mission to seduce Yugoslav President Marshal Tito, a nebbish diplomat must smuggle Marilyn Monroe, the US?s biggest movie star and newest spy, out of Yugoslavia before their blunder leads to all-out war.

Good idea; good title, messy logline.

I feel first and foremost, you need to trim this down. Keep it focused on your protagonist – what is the diplomat’s flaw?

By spending so much time on the setup, the logline becomes unbalanced, and it feels like the protagonist is going to Marilyn; until you assign the action and goal to the logline.

The event always needs to be something that happens TO your protagonist. It can’t be something that he does to set the story in motion – so him botching the mission can’t be it. What CAUSES him to botch the mission?

Perhaps:
When their cover is blown on a mission to (what was the end goal of seducing Tito?), a nebbish diplomat must smuggle Marilyn Monroe, the US’ biggest movie star and newest spy, out of Yugoslavia before their blunder leads to all out war.

I can see Woody Allen and Scarlett Johannsen in the lead roles ….

I see you’ve made the changes I suggested – which is very flattering! My question is now … what would they hope to gain by seducing Tito? This would be the overarching goal, I assume, and likely to be higher stakes than just seducing him?