Hank is outside putting together a giant sloth skeleton the X-Men received as a gift from the people of the City of Mists. Bobby is assisting by providing outdoor air-conditioning, Jean by lifting pieces with her telekinesis, and Scott supervising the situation.
Xavier predicts that the build will result in trouble, and thus has Hank finish it outdoors before bring it inside the mansion. An impatient Bobby insists on fitting the head despite Hank’s disapproval because he has not put in “a brace under the neck to support it”. Bobby causes the whole skeleton to collapse and Hank has to start over.
Xavier is on a call with Dr Stack, and he watches Hank chase Bobby around the mansion. He is expecting a visit tomorrow, and the X-Men speculate that Xavier is recruiting another mutant to join the team. The X-Men are reacting to the notion that Warren might be replaced. Jean is upset. Bobby is optimistic that Warren will come running back. Scott and Hank are willing to accept a new team mate if he “shows promise”.
The X-Men have Hank spy on Xavier’s conversation with Dr Stack and someone who appears to be a prospective student. The “student” overhears Hank and attacks Hank. Bobby, Scott and Jean jump in to help Hank. Xavier holds back Dr Stack, wanting to watch how his students react.
After a few minutes, Xavier commands his X-Men to stand down. Dr Stack was introduced to Xavier through FBI Agent Baker. Dr Stack wanted to acclimate his artificial intelligence X-Series robot (built for the military), named Aaron, to real world situations.
Scott and Jean are in the kitchen preparing lunch while Hank and Bobby converse with Aaron. Jean is upset that Xavier is letting Aaron join the X-Men, even though Scott reminds her that Aaron is just visiting briefly and not replacing Warren. Aaron overhears and senses Jean’s hostility towards him. Hank explains that Jean is taking it the hardest with Warren’s departure from the team and she is not ready for a new team mate.
Aaron gets distracted with Hank’s skeleton project and proceeds to analyse and complete the skeleton in mere seconds, while Hank tries to educate Aaron on the etiquette of not eavesdropping. Hank is astounded by Aaron’s wealth of scientific knowledge. Lunch is served. Aaron tries to confront Jean about her hostility towards her, which comes out as a misunderstood paraphrase of Bobby’s earlier statement. Scott offers Aaron some water since he does not process food. Xavier announces that Aaron will join the X-Men for an “investigation” after lunch.
They arrive at the Mount St. Helens National Volcano Monument. Agent Baker is there to meet them and brief them on the situation. It has been reported that the smoke coming from the volcano resulted in ice falling into a collapsed dome in the caldera, and nothing out of the ordinary. But Agent Baker believes that something suspicious is happening in the area since the smoke as there have been recorded sightings of “lava men”, similar to the one Thor defeated some time back. Agent Baker has been telling the locals that the sightings are merely hoaxes, and hope that the X-Men will investigate the truth to the sightings.
Meanwhile, the X-Series base in the military is experiencing a malfunction in the absence of Dr Stack. The robots are revolting against the military considering them to be mere machines to follow orders.
The X-Men and Aaron set off into the forest. Aaron displays his ability to “negat[e] gravity to scan from a higher point”. Jean hammers in the fact that since Aaron is able to fly, which was what the rest of the team considered Warren to be. Scott groans that Jean is starting the banter again, when Aaron alerts them to “geothermal readings moving to [their] location from below”. The X-Men find themselves ambushed by an army of “lava men”.