What if the Enterprise came to visit present-day Earth? Look no further than Tomorrow Is Yesterday for your answer. Of course, time travel has rarely been done, and it's not a very well understood procedure, so the Enterprise has to approach the whole thing with kid gloves, which means some interesting hijinks will ensue. The episode's best material involves Captain Christopher aboard the Enterprise, and Captain Kirk at the airbase. Christopher's situation isn't too funny, and provides some dramatic weight, but Kirk's is plenty hilarious, ensuring that this episode has lots to offer. A great scenario, well executed.