On board the Enterprise, the Universal Translator has resulted in a minor diplomatic incident with the Kreetassans. As the Kreetassan vessel departs, a clear, amoeba-like entity crosses to the Enterprise. Later, Hoshi analyzes the Kreetassan language and finds that each word can have a dozen meanings depending upon its context. She also hears the entity moving on the other side of the corridor-inspection-plates, but decides that it is nothing of importance.
Systems however begin to malfunction on a ship-wide basis. Crewmen Rostov and Kelly are both trapped in a cargo-hold by the entity which has now grown tendrils. Captain Archer, Commander Tucker, Lt. Reed and security officer Zabel go to investigate and are all caught as well, except Reed who escapes through the cargo-hold-hatch, severing one of the tendrils in the process.
Ensign Mayweather suggests tracking the Kreetassans down to ask about the entity, while Dr. Phlox examines the severed tendril. Hoshi wants to communicate with it, but Sub-commander T'Pol decides to neutralize it instead. Phlox determines the entity is photosensitive, so light might stun it. However, the attempt fails, causing pain to the trapped personnel. They learn that those entrapped are becoming symbiotically linked together through the entity.
Mayweather manages to find the Kreetassans and learns that they are now able to communicate. Mayweather asks the leader for any information they may have about the entity. He tells Mayweather that they know the location of the entity's home world and agrees to transmit the coordinates, but only after Mayweather apologizes for the earlier incident. Apparently the "misunderstanding" occurred when the Kreetassans were taken to the mess-hall to find many of the crew eating, since the Kreetassans do not eat in the presence of others, comparing it to the mating-process.
T'Pol, Hoshi and Reed all finish their research and make their way to the cargo-bay to attempt communication. Reed assembles experimental force-field-emitters, which are able to protect them from the entity's tendrils. Hoshi uses the Universal-Translator to modulate a frequency that the entity can understand. After several attempts, the entity responds. It gives them new, more precise coordinates on its home-world, and Phlox notices that the bio-signs of the trapped personnel are stabilizing. The communication ends and the entity releases the personnel, shrinking back to its original size.
On the entity's planet, T'Pol, Reed, Phlox and Hoshi land in a shuttle-pod and release the entity at the coordinates given earlier. Phlox also releases the tendril severed in the cargo-bay. Both organisms quickly rejoin and are re-absorbed into the larger alien body. As the shuttle-pod returns to the Enterprise, dawn breaks and the entire area is revealed to be covered with one huge organism.