Magnus returns to the Sanctuary from a conference expecting to meet with Dr. Lily Lee, a member of the UN Security Council, but finds, instead, a facility that looks like it's been through a natural disaster. If that weren't unnerving enough, no one from her team is anywhere in sight. After carefully picking her way through the wreckage, she finally finds Henry, bloodied and bruised and with virtually no memory of how he got that way. The last thing he remembers is cataloguing the new intake, called a mollox, a normally docile creature that freaked out and attacked him. Henry tries to access the Sanctuary's security footage (he's been locked out, for some reason) while Magnus goes to look for the others.
She finds Kate, bound in a closet, and suffering from the same amnesia as Henry. Soon they encounter Will, and later the Big Guy, and slowly piece together a rough timeline of all comrades, at various times, attacking one another, from simple punches (Kate) to rigging the facility's security systems to attack Will (Henry). Magnus runs blood tests and finds no evidence of a pathogen, so someone or something must have skipped into the Sanctuary undetected, and it's making everyone turn against each other.
After slogging through the video footage, Magnus finally sees it: An abnormal called a pinnepex, the size of a mouse (and ugly as sin) has been using the Sanctuary team as hosts, jumping from one to another depending on who was strongest. This little gremlin is somewhere in the Sanctuary, but to complicate things, so is Dr. Lee. They deduce that, since none of them are in attack mode, the pinnepex must have attached itself to her.
Magnus whips up a pheromone that will repel the wee beastie, and the team goes hunting. They find Dr. Lee (affected, as suspected), flank her, and take her down.
The amnesia wreaked by the pinnepex was a liability a few hours ago, but now it seems to be a boon: Magnus and the team set up an alternative version of the truth to tell the good doctor, so that when she wakes, she doesn't need to know (or report to the UN) about her adventure with a parasitic abnormal. They'll tell her she fainted, no one but the pinnepex will be any the wiser, and the Sanctuary will be allowed to go on ticking.