Season 3, Episode 11:
Reciprocity
Olivia, Peter and Walter are brought to the Massive Dynamic laboratory where the Wave-Sink Device has been assembled. No one has been able to access any of its functions, but when Peter gets close to it, the machine actives of it's own accord and Peter suffers from a nosebleed.
Soon after, Fringe Division is called in to investigate a series of murders after the victims are discovered to be Shapeshifters. Broyles and Olivia believe that it's the work of another Shapeshifter, cleaning up the network to stop Fringe Division from capturing and interrogating any of their number, and that cracking the encryption on Fauxlivia's laptop might help them track down the rest of the Shapeshifters in the prime universe. However, it's Walter who is first to discover the truth: the killer is Peter, who already cracked Fauxlivia's laptop and is recovering their data discs. After figuring it out, Walter arrives too late to stop Peter from killing the last Shapeshifter, but doesn't give him up to Olivia and Broyles.
Later, Peter claims that he is attempting to be proactive in the fight against the alternate universe, and that Shapeshifters aren't real people, but Walter theorises that the machine has somehow "weaponized" Peter.
TROPES:
- Artistic License – Medicine: What is the point of putting two ECG leads on Peter's head when he goes into the scanner. I know he is from another universe, but I don't think his heart is in his head and that certainly is not enough electrodes to do an EEG.
- C.A.T. Trap: Walter says the scanner will expose Peter to radiation suggesting that this might be a CT or PET, but those would not give us information about electromagnetic activity. This could be a MEG, but that would not involve radiation.
- LEGO Genetics: A retrovirus based on chimpanzee DNA that cases Walter to crave bananas and make shows of dominance.
- Loyal Phlebotinum
- Psychic Nosebleed
- Shout-Out: We find out Fauxlivia's password is a U2 lyric, which is also a Continuity Nod to how she was going to go to a U2 concert with Peter in "6955 kHz" and found Bono attractive in "The Box".
- Technopath
- What Measure Is a Non-Human?: Peter's attitude towards the Shapeshifters. Walter calls him out on this. On the other hand, nobody cares for the collateral damage in form a mercury-polluted pond full of dead fish.
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