Complex sentence: Meanwhile, the children and lead Groske determine that for the Shansheeth plan to have worked, they needed help from someone in UNIT, and, using the air vents, learn that Karim is working with the Shansheeth.
Split into simpler sentences: Meanwhile, the children and lead Groske determine that for the Shansheeth plan to have worked, they needed help from someone in UNIT. Using the air vents, they learn that Karim is working with the Shansheeth.