Chekov, having been massively aged to the point of near-death by radiation-induced disease, is called up to the bridge to man the weapons console of the helm and defend against a hostile ship. He utterly annihilates the enemy ship while gracefully dodging the Enterprise around its shots. All that buildup throughout the episode and Chekov really delivers. Of course the episode's Stinger reveals it was all a dream, it was still awesome. Bonus points for having Walter Koenig reprise his role.