I don't get this specifically: When some people are up to something.
For example: We’re up to the murder.
It occurs in this passage from The Lost Girl: A Fear Street Novel by R. L. Stine:
“You don’t mind if I clean my jewelry while you work, do you?” Mom asked. She sat down at the other end of the table and started pulling rings and earrings out of the box.
“We’re up to the murder,” Gabe said, reaching for a blank sheet of paper. “What if we make it so the player can choose his victim?” Diego asked.
“You know. Like who should Macbeth kill first? Maybe we give him an automatic rifle, and he runs through the castle—”