- Dr. Yui Sasaki (Main story and flashback)
- Hellfire Club's Inner Circle
- Mastermind (Jason Wyngarde) (Impersonated as Jun Sanada)
- Riko Nirasaki
- Koichi Kaga
- Cyclops' Visor
- Analyzer Visor
- Anti Damon-Hall Syndrome Serum / Anti Viral Drug
- Cerebro (Mentioned)
Dr. Sasaki comes under attack by a monstrous bug. The X-Men come to her rescue, badly wounding it. Once defeated, it expels some smoke and it's remains reveal that Rika had entered a second mutation. Koga runs off in fear. Treating Rika, Beast finds track marks on her arm; Sasaki said its due to Rika being diabetic. Wolverine and Cyclops track down Koga, who they know is a mutant; he injected himself with the mystery serum. However, Koga soon mutates as well; he is knocked out by the X-Men. Dr. Sasaki is forced to reveal she created a serum to block mutation. Unfortunately, in its current state, if the user's body adapts to the serum, the mutation will snap back with ferocity; Rika and Koga each sought out Dr. Sasaki for help returning them to normal.
At the same time, Emma tracks Jun to a sub basement and reveals that she knows he's really Mastermind in disguise. His ruse found out, Mastrermind reveals he tracked a great power to this facility. He even went so far as to trick the U-Men into giving him the information he needed.
- Original airdate: May 27, 2011
- English airdate: December 9, 2011
- Wolverine mentions the Brood only in the english version.