The series, set in the shared Marvel Universe, is part of the wider X-Men franchise. The titular X.S.E. is "Xavier's Security Enforcers", a mutant police force that almost a century into the future, founded to keep order after humanity and mutantkind reclaim their freedom from the robot Sentinels.
Bishop is a time-travelling mutant X.S.E. officer who was trapped in the present day after pursuing a time-travelling supervillain, then ended up joining the X-Men.
Shard, Bishop's younger sister, was another X.S.E. officer, and eventually her brother's superior. She died in the future, but an electronic copy of her mind is now connected to a hologram and she's also back in the present day, working with the X-Factor team.
Using a conversation between the siblings as a Framing Device, X.S.E. tells a story about their shared past, their lives in the future - and exactly how Shard died...
X.S.E. contains examples of the following tropes:
- Brain Uploading: Shard's mind is uploaded after the Emplates convert her.
- Framing Device: In the present, time travellers Bishop and Shard are talking about their past, almost a century in the future. Everything else is a Whole Episode Flashback showing how they got there - and how Shard died.
- Too Many Mouths: The vampiric Emplates have fanged mouths in the palms of their hands.
- Whole Episode Flashback: The series uses Bishop and Shard's conversation in the present as a Framing Device, but all of the plot takes place in the future.
