Calling all Heroes!

So, here is one of the more important questions I was going to address, and the one that started me rethinking my entire module design. Why do we need the heroes?

At first the module was going to just be about the fall of Mishann's Crown, a small town on the frontier of Cyre. The Brelish PCs were supposed to stall Cryan forces before finding a way through the town's defenses and finally taking the town itself. Or so they thought. Unfortunately a greater force of unknown foes was also making ready to storm the town, and wipe out any opposition around them. The PCs would then have to escape this final confrontation with their lives and make their way back to friendlier territories. That's all well and fine (and the PCs will still have to accomplish all of this), but it shouldn't be the main focus of the adventure, because we already know it has to happen.

So what is going to make the heroes be... well, heroic?