The Enterprise runs into a mysterious energy field of a type it has encountered several times before.
The Enterprise goes to visit a remote outpost of scientists, who are all perfectly all right.
Someone visits the holodeck, and it works properly.
The crew of the Enterprise discovers a totally new life form, which later turns out to be a rather well-known old life form wearing a funny hat.
The crew of the Enterprise is struck by a mysterious plague, for which the only cure can be found in the well-stocked sickbay.
The Captain has to make a difficult decision about a less advanced people which is made a great deal easier by the Starfleet Prime Directive.