Seven of Nine must run the ship alone.
Voyager must cross a nebula that is dangerous to most of the crew, and the senior officers are put into stasis, leaving Seven of Nine alone to guard the ship. She has to cope with isolation, and the Doctor must help her face the threats.
The episode is a strong character study for Seven of Nine, who is an expert in being part of a group, and now must face what it means to be alone. It also provides a chance to see the Doctor and Seven working together, and it builds a sense of tension.