What element of science fiction stories are you comparing if you look for how the events are alike?

The plot.

Explanation:

A plot can be defined as a literary device which is typically used to describe the sequence of events that make up a literary work such as story, film etc., and how they affect one another through a phenomenon known as cause and effect.

This ultimately implies that, the structure of a literary work solely depends on the organization (pattern) of events in the plot of the literary work.

Hence, the plot is an element of science fiction stories that are used for comparing how the events are alike because it is building blocks (foundation) around which the setting and characters are developed.