Henry and Declan bring a team to a Russian icebreaker to rescue the crew and Abnormal passengers on that ship and solve the mystery of why it ran aground. On arrival, however, they discover all the crew members dead and some very heavy, unexpected weaponry. As they begin puzzle-solving, Magnus and Will arrive in the midst of a ferocious blizzard, and the Sanctuary party, now numbering seven, discover a new element: Frozen blocks containing an unidentified Abnormal life form.
When they decide to partially thaw one of the blocks in order to do a biopsy, they discover that the creatures in the ice are magoi, the deadly shape shifters that nearly wiped out Will and Magnus as they transported Abnormals across the Himalayas. These beings achieve their ends by manipulating human psyches, often with deadly results. If the Sanctuary team isn't careful, they could wind up turning against one another in a paranoid hallucination.
Salt water begins filling the lower holds of the ship, where most of the magoi lie frozen. Magnus has told the team that salt water will supercharge the abilities of the magoi, so they devote all their resources to moving the blocks above board and out of harm's way. But as they do so, bodies pile up - first a xenobiologist, then her fiance, Alistair, a HAP whom Henry recruited after he left the sham institution formerly run by Erica. The third member of Declan's team is also felled, but Magnus won't let them pause to grieve until all the magoi are out of the path of the encroaching salt water.
Once this task is completed, Declan and Henry go to remove Alistair's body from the pool of salt water in the hold, but while there, Will appears and locks them in. They manage to escape by breaking the psychotic illusion the magoi placed on them - he made them believe Will locked them in - but when they find Magnus, she tells them she got separated from Will, and the trio must go looking for him. Just as they reunite with him, two more visitors arrive - another Will and Magnus! Henry and Declan, who were soaked with salt water dragging Alistair's body from the hold, know they can't be magoi, which means that one pair of these identical twins must be the deadly creatures.
The Magnuses trade facts and insights about magoi, each trying to prove their claim at authenticity, but when the second Magnus proposes taking a step into the salt water, tensions ratchet higher. Declan and Will fear the prospect of a super-charged magoi, but Magnus states that this is folly - salt water kills magoi, it doesn't help them. Declan and Henry have their fingers on hair triggers as Magnus lunges at her double and pulls them both into the water, thus killing the impostor. The other magoi tries to escape, but Will shoots him as he flees. A sodden Magnus emerges from the chilly salt water and begins the difficult clean-up. Her team, badly shaken by recent events, want to leave the magoi behind, but she insists on the innocence of the still frozen beings and arranges for their secure transport to the Sanctuary.