With episode four being a bit on the slower side, this week's installation of Halo brought in some much-needed action on the battlefield while still working on the plot with Chief and the keystone.
On Eridanus II, we're visiting the Reach for Life Installation, which is basically a nature-based school for children. With all of the kids in uniforms, it basically looks like a weird child cult. However, it turns out this is a bit of earlier Eridanus II, as we're following a much younger Dr. Halsey, who seems to be touring the school (slyly scouting for kids).
They're basically in a large, fenced-in area where they can explore gardening, climbing, and other outdoor activities. A young Master Chief is up above on some precariously raised platforms and is being teased by another boy. However, when the boy almost falls off, Chief is able to save him from a deathly fall by lifting him up, which is seen by Dr. Halsey. She's impressed by the school and asks her husband what he thinks; though he outwardly seems interested, it's clear that he's not so sure about Chief. At that moment, we cut to the present, showing this man as one of the captains of the UNSC.
Dr. Miranda Keyes arrives on Eridanus, as well as Kai and the other Spartans, who are setting off to join Chief. It turns out that the man from before is her father, Captain Keyes, and they head off to a temporary UNSC camp to find Halsey and her new discovery: the second keystone. Cortana informs Chief that this keystone has an energy power much greater than the original one found on Madrigal. Upon hearing that, Halsey directs him and the others to stay away from it, as its power could be potentially dangerous.