The Doctor and Seven of Nine discover that a mysterious illness that is striking down the crew one by one is actually the work of enigmatic aliens who are using the ship for genetic experiments.

Annette Helde (Takar)
Rosemary Forsyth (Alzen)

• This episode takes place three days after the events in "Day of Honor".
  
• Tom and B'Elanna's relationship escalates....
  
• The crew's DNA is being manipulated and mutated by aliens who are slightly phased. They are doing scientific "research" in order to help their own society live better and longer.