Upon investigation, a search reveals no-ones aboard. Spock analysis indicates the dead crew of an insectile race of engineers destroyed themselves but the question is why? Now for the fun bit. Yes, theres an alien life form and its still alive even after all this time!
"You will obey me!"
When the creature beams back to the USS Enterprise 1701 it invades the main computer (much like V'ger did in Star Trek TMP) pushing the bridge crew to the edge. The mind twister here is how to get the alien entity off the Enterprise! Time for a little Captain Trickery to level the score? You decide.
Who said "Its a starship, nothing I've ever seen, look at the size of it!!!"
If haven't seen Star Trek TAS before then you'll likely be taken in by the shows detailed visual imagery. The series won a daytime Emmy award for "Best Children's Series" in 1975.
Slingshot long and prosper, trekkers.