In the episode, the Fringe Division of the parallel universe investigates a series of deaths caused by flesh-eating "skelter beetles", unleashed by a mad scientist (Alon Abutbul).

Abutbul, Seth Gabel, Kirk Acevedo, Philip Winchester, Ryan McDonald, and Joan Chen appeared as guest stars.

The episode was written by co-executive producer David Wilcox and story editor Ethan Gross, and was directed by filmmaker Brad Anderson.

Time shifted viewing led to an addition of 64 percent to the week's original ratings, but lead actor Joshua Jackson still expressed concern for the show's future, urging fans to watch the series live on Fridays.

Television critics viewed the episode positively, with one praising it for offering "a creepy case, a great villain, and a pertinent plot twist".

[1] In the parallel universe, Lincoln (Seth Gabel) has been promoted to head of the Fringe Division, while Fauxlivia (Anna Torv) welcomes back Frank (Philip Winchester), her boyfriend who has been working as part of the Centers for Disease Control to deal with an outbreak in North Texas for the last several weeks.

When he hears of Fauxlivia's pregnancy with his grandchild, Walternate comforts her and offers his complete support for her, believing the child to be another way to bring Peter voluntarily back to the parallel universe.

"Immortality" was written by co-executive producer David Wilcox and story editor Ethan Gross, and was directed by frequent Fringe collaborator Brad Anderson.

[3] The episode's premise was based on an extinct type of beetle, which caused trouble for guest actor Kirk Acevedo, as he had a fear of bugs.

"[5] In addition to Acevedo, other recurring guest actors included Seth Gabel as Lincoln Lee, Philip Winchester as Frank Stanton, and Ryan McDonald as Brandon Fayette.

The first was the presence of Joan Chen, who starred in the role of Reiko, Walternate's mistress, and had appeared on Twin Peaks as the femme fatale Josie Packard.

The second allusion is made when the second victim orders "a piece of cherry pie" at a diner, alluding to the Twin Peaks lead character Dale Cooper's own fondness for this item.

[14] "Immortality" was watched by about 3.7 million viewers, drawing a 1.4/4 ratings share on its initial broadcast, down 12.5 percent from the previous episode's viewership.

[1] Andrew Hanson from the Los Angeles Times' enjoyed the "fun" scenes in the parallel universe, and stated he would not mind watching an entire show about their Fringe Division.

The storyline with the Dr. Silva [sic] and his beetles didn't hold my interest at all, but everything else in this installment was fascinating material that expanded the mythos of the series far more than I could have imagined.

"[20] Den of Geek's Billy Grifter wrote, "What this show is now quite good at is lulling us into a false sense of security, before pulling away the rug rather smartly.

If you follow the show, like me, you probably got a slightly déjà vu feeling to the plot, as it rode the previously saddle worn concept of a scientist who has lost the perspective to realise that he's killing people for science.