The introduction scene follows from "Jacksonville", where Olivia (Anna Torv) has discovered, through her Cortexiphan-induced abilities, that Peter (Joshua Jackson) is from the parallel universe, and Walter (John Noble) takes her aside to tell her how this came to pass, shown to the viewer in an extended flashback.

In 1985, Walter and William Bell had theorized the existence of a parallel universe, and created a window-like device to observe it.

Though they use their observations for military benefits, Walter has a more personal interest in the parallel universe, to seek a cure for a genetic disease crippling his son Peter (Quinn Lord).

Walternate is distracted by the arrival of an Observer, September (Michael Cerveris), and fails to see the telltale color change indicating a cure.

Nina attempts to tackle Walter as he steps through, but instead part of her arm disappears in the portal as it closes; Carla rushes her to the hospital.

At that point, Walter realizes he will never be able to make himself return Peter to his proper universe—the pain of losing their child for the second time would be too great.

In their report, they were looking for a woman roughly forty years old who was described as being "strong (yet broken), intelligent, attractive, likable, lovely and extremely versatile".

[3] In December 2009, Entertainment Weekly announced Irish actress Orla Brady had been cast as Elizabeth Bishop, a possible recurring character.

[4] Some critics questioned the casting, as they considered Brady to be too young and were unsure if she would appear in the present or a flashback sequence.

"[6] The production crew tried different methods with the actor to create a younger Walter, such as changes to his hair, make-up, and wardrobe;[6] they ultimately decided on using "tabs" to keep the wrinkles in his face back.

[10] As with other Fringe episodes,[11] Fox and Science Olympiad released a lesson plan for grade school children based upon the science depicted in "Peter"; the lesson's intention was for "students [to] learn about the challenges of effectively communicating scientific principles and concepts, especially via non-verbal methods.

[15] In a Thursday night that had rating slumps from all the network shows, "Peter" was down 15 percent from the previous episode "Jacksonville".

[17] Tim Grierson of New York Magazine also praised the emotional performance of John Noble, calling it his "showcase" and one where "he didn’t disappoint".

Club's Noel Murray considered the episode one of the best of 2010 up to that point, and that "the show came back with a sense of purpose and surety it’s rarely had, even at its best".

[22] Similarly, BuddyTV rated it the seventh best episode of 2010, praising it for "open[ing] the door to the show’s brilliant use of the alternate universe," and calling Noble's acting "one of the best performances on TV".

[23] SFX ranked "Peter" among the twenty things they love about the series, calling it "the lynchpin episode on which all of Fringe rests.

As one result of the high praise and fan appreciation for "Peter", a future episode, "Subject 13" (originally named "6 Months Later") in Fringe's third season aired on February 25, 2011.

The episode was marked by a pitch-perfect pace, excellent "youthenizing" special effects, and a story that plays your heartstrings like an expert bard plucking out a tearful tune on a lyre.

Wyman, Jeff Pinkner, Josh Singer, and Akiva Goldsman submitted "Peter" for consideration in the Outstanding Writing for a Drama Series category at the 62nd Primetime Emmy Awards.