Part A: "The last time he and Trace had met, Trace and his friends had thwarted Casper's plot to create a deadly toxin designed to help him take over the world."
Part B: It gives readers insight into who Casper is and how the group knows him.
Explanation:
"The last time" indicates that what they tell you next will have taken place in the past. The section after that explains who Casper is and why he is their enemy. Hope this helped !!
