The narrative followed the Fringe team's attempts to extract William Bell from Olivia's brain by entering her mind with the help of LSD.

Production of the animated portions was completed by Zoic Studios in an eight-week effort, the largest amount of work hours required to produce a Fringe episode.

An estimated 3.6 million viewers tuned in, helping the episode earn a 1.4 ratings share for those 18–49, tying a series low.

The mind of William Bell (Leonard Nimoy) still possesses Olivia's (Anna Torv) body after several failed attempts to extract it to recently deceased corpses.

They realize that Olivia is unaware that she has been possessed by William's mind, and instead has likely locked her ego away, making it difficult to contact her by normal means.

Walter sees someone sending a Morse code signal from William Bell's office in one of the World Trade Center buildings.

After evading a crowd and a trap set by a false vision of Nina Sharp (Blair Brown), they arrive at Bell's office, where they find William waiting for them as an animated cartoon.

Peter realizes that when Olivia is scared, she retreats to somewhere safe, and suggests they search her mind's version of Jacksonville, her childhood home.

They are soon attacked by a man (Ulrich Thomsen) wearing an X-marked T-shirt, who tears open the side of the zeppelin; Walter is pulled out by the rush of air and falls to his death—waking him back in the real world.

[1][2] "Lysergic Acid Diethylamide" features the return of guest actor Leonard Nimoy to television, as he had previously announced his intention to retire from acting.

[3] Nimoy stated that when the writers approached him about the role, he jokingly commented on having experience playing characters returning from the dead—referring to the death of Spock in the Star Trek movies— but expressed interest because of his appreciation of the show and its writing in general.

Part of our plan has always been to get Walter to embrace his flaws and uniqueness as strengths, rather than thinking of them inhibiting his performance as a scientist and as a character.

"[9] Because the season finale did not resolve the plotline of Olivia and the man from the blimp, Pinkner commented in a post-finale interview that "It’s definitely still in play.

[12] To celebrate this, Fox began a promotion the day "Lysergic Acid Diethylamide" aired, lasting four weeks until the season finale.

[17] "Lysergic Acid Diethylamide" was watched by 3.6 million viewers on initial broadcast, with a 1.4 rating in the target 18–49 age demographic, tying its previous season low.

SFScope Fringe reviewer Sarah Stegall noted that the episode's ratings were not surprising, considering the series had just returned from a three-week break.

Bold and daring, funny and shocking, it challenges audiences to follow a complicated plot through visual changes like turning the show into a cartoon for half the episode, showing us a tripped-out Broyles blowing bubbles, and offering us references to genre movies like Inception.

Writing for the Los Angeles Times, Andrew Hanson questioned whether the writers had been using drugs lately because of the surge of drug-related episodes; though he loved it, he acknowledged that the episode was one "you are either going to love or hate", explaining that it "got a little crazy using Olivia’s mind as a setting, [but] it stays grounded through the emotional current running underneath... you have to give Fringe credit for taking a risky, bold move.

He loved it for several reasons: he is a big fan of Anna Torv's Bellivia performance and its plotline, the drugs incorporated into the plot, its Inception-like qualities, the animation, and Bell's final goodbye.

Like Murray, Isler also noted the similarities to Inception as well as to the film Yellow Submarine (1968) and the science fiction novel A Scanner Darkly (1977).

), buttoned-up Broyles loopy reaction to LSD exposure, and the mysterious Max X (now believed to be a riff on Bell), the Zeppelin saboteur who Olivia predicted would one day kill her.