In the episode, the fringe team investigates mysterious deaths at an apartment building, the result of a merging between the prime and parallel universes; while there, they encounter a woman who claims to be able to see the ghost of her deceased husband.

The episode featured appearances from recurring actors Ryan McDonald and Seth Gabel as well as one-time guest stars Phyllis Somerville and Ken Pogue.

When the Fringe team investigates, Peter (Joshua Jackson) and Walter (John Noble) determine by the location of the bodies that they seem to have fallen through the balcony, as if it momentarily ceased to exist.

Olivia notices a similar glow emanating from the windows of apartment 6B in the Rosencrantz building, owned by the widow Mrs. Alice Merchant (Phyllis Somerville).

If the crack widens, Walter predicts they would see occurrences of the same singularities that have plagued the parallel universe, and suggests the use of the same amber-like compound they had previously recovered ("The Ghost Network") to limit the damage.

As the building starts to exhibit the initial signs of a singularity and Broyles (Lance Reddick) prepares to release the amber, Olivia and Peter attempt to persuade Alice that the man she sees is not really her husband.

In the parallel universe, Fauxlivia and Lincoln (Seth Gabel) investigate reports of a Fringe event in the Rosencrantz building, and speak with Derek, who tells them he has not noticed anything out of the ordinary.

The episode was co-written by story editors Glen Whitman and Robert Chiappetta, while being directed by cinematographer and Fringe director of photography Thomas Yatsko.

"[5] "6B" featured recurring guest actors Ryan McDonald and Seth Gabel as Massive Dynamic scientist Brandon Fayette and Agent Lincoln Lee, respectively.

[14] Entertainment Weekly staff writer Ken Tucker thought it was not "the best Fringe episode", primarily because of "its Twilight Zone construction, with a supernatural event serving as a big, obvious metaphor for... the rift between lovers (Peter and Olivia; the elderly couple Alice and Derek)".

[1] In her 2011 book Into the Looking Glass: Exploring the Worlds of Fringe, author Sarah Clarke Stuart found parallels between the episode and the 2011 Tōhoku earthquake and tsunami, which occurred several weeks after "6B" aired.