The protagonist must be inescapably tied to the central unifying problem, and compelled by circumstances to solve it. Express the action of your story as a verb. The action is toward this goal. The goal should be compelling and the reader should want the protagonist to accomplish it. It must be a viable and riveting complication to the protagonist's story.
Vulnerability starts the process working.
Altruism deepens the process and brings it to fruition. The greater the adversarial force, the more dynamic the tension. If adversary is a villain, it helps if the character is in some way sympathetic, so the reader becomes involved in that character's downfall or failure. Throughout the story the protagonist and adversarial force must remain irreconcilable opposites. For whenever the conflict between them ceases, immediately all the energy dissipates and the tension disappears.