2 ideas
Edgar, an aging artillery captain, and Alice, a former actress who gave up the stage to marry him, live in a fortress tower on an island, hated by the garrison and nearing their silver wedding anniversary. After all of it, the couple decide to 'cross it out and go on,' and resume their marriage as though nothing had happened.
In a relationship of mutual destruction, the fighting is not a sign the bond is failing; it is the bond. Each partner's identity depends on having the other to defeat, so any outsider brought in as an ally or lover becomes ammunition. Once the crisis passes, the pair close ranks and the outsider is discarded, because leaving would mean losing the enemy that gives each of them a purpose.