When a businessman sells up to recover from debilitating injury, he embarks on an apparently simple quest to trace the origin of a family heirloom.

Hi Russel!

Just go back to formula. The incident in something that happens to the character. So, the injury? So more clearly, the need to pay a steep medical bill?

Character: a businessman. You could give us a juicy character flaw, here.

Action: ascertain value

Maybe something like:

When he can't afford a medical bill, a loner (?) businessman must connect with his outrageous relatives to prove the origin of a valuable family heirloom.

Obviously, replace my suggestions, with what makes sense for your story. :)

I don't see? a clear cause-and-effect relationship between the injury and the quest for the heirloom.? ? And? there is no "must"? in the plot.? No compelling reason, no sense of urgency as to why he must find the heirloom -- or else.?

Or else:? what's at stake, what does he stand to loose if he doesn't find the heirloom?? ?What does he stand to gain if he does?

(And should it be "sell out" rather than "sell up"?? That phrase needs clarification.)

I don't see? a clear cause-and-effect relationship between the injury and the quest for the heirloom.? ? And? there is no "must"? in the plot.? No compelling reason, no sense of urgency as to why he must find the heirloom -- or else.?

Or else:? what's at stake, what does he stand to loose if he doesn't find the heirloom?? ?What does he stand to gain if he does?

(And should it be "sell out" rather than "sell up"?? That phrase needs clarification.)