Base's supplies are sabotaged. At Portlaoise, Arthur gets a letter from a secret admirer "XX". Distrust mounts over the tampering.

Chaos strikes the base as the trial's computers are infected with ransomware at the same time the participants suddenly begin to get sick. With trust at an all-time low, things come to a head when the root of all evil seems to be unmasked. Meanwhile, Arthur carries out Patrick's dirty work in exchange for access to a cellphone.

After a forceful shift in power, both scientists and subjects do whatever it takes to stay alive -- even if it means turning on one another. Following a jarring discovery, the remaining subjects attempt to escape the facility, while, at Portlaoise, Arthur receives an unexpected visitor.

When Rachel goes into labor, a brief truce occurs with Maggie, who helps deliver her baby. Shortly after, an unexpected downpour saves them both from dehydration, but only briefly, as the killer ultimately strikes again. Later, the identity of XX is revealed.