All the World's a Stage

The Protostar crew responds to a distress call from an M-class planet, except for Murf who is unwell. They discover an alien race called the "Enderprizians" who revere and imitate Starfleet. A mysterious disease is afflicting the villagers, and also infects Dal. They track the disease to a cave holding a Federation shuttlecraft that crashed on the planet with Ensign Garrovick of the USS Enterprise long ago. The shuttlecraft is leaking plasma and poisoning the environment. The crew and villagers work together to dispose of the shuttlecraft and seal the cave while Zero devises a cure. When the crew return to the Protostar, they find Murf encased in a cocoon.

Summary from Wikipedia, CC BY-SA 4.0.