Norwegian Ninja is the true story of how Commander Arne Treholt and his Ninja Force saved Norway during the Cold War. In 1983 the Ninja Force discovers that the sinister NATO force Stay Behind, who take charge in times of war and emergency, are planning a coup-d'état in peacetime. Treholt and the Ninjas see only one solution: a full can of whoop-ass.

Sarah (Ann Eleonora Jørgensen) and David (Magnus Crepper) meet for the first time in ten years when she arrives at his summer residence. David needs help finishing a play, which turns out to be a dramatization of their long, troublesome relationship. The film presents three love stories about seemingly different couples; all named Sarah and David, but played by different actors in respectively their 20s, 30s and 40s.