The Mantis-Eye Experiment

Season 5, Episode 7: "Bot Seeks Bot"

While Vendata/Venturion was technically introduced as Councilman 1 way back in season three (and shown in silhouette a few times after), "Bot Seeks Bot" is the first episode where we see him in full and at least part of his backstory is explained. All of the early flashback scenes that take place in the recent future are take directly from "Bot Seeks Bot".

This episode also confirms that Vendata is the Blue Morpho and the Monarch's father.

Season 3, Episode 3: "The Invisible Hand of Fate"
Season 4, Episode 1: "Blood of the Father, Heart of Steel"
Season 5, Episode 3: "SPHINX Rising"

While in the diner Brock is told the truth about the OSI, SPHINX (SPHINX!) and the Pyramid Wars. Most of this was briefly introduced in the season three episode, "The Invisible Hand of Fate" in a scene where Brock and Hunter are investigating the shadowy and still unknown Guild of Calamitous Intent while the rest of OSI brazenly fights (or really slaughters) SPHINX in the Pyramid Wars. This is also touched in on the season five episode, "SPHINX Rising" where 21 accidentally recruits former members of the original SPHINX to be in his new, good guy version of SPHINX (which he gets from Hunter when Hunter is promoted to head of OSI). "SPHINX Rising" also contains the first evidence that Dr. Venture and the Monarch's parents knew each other, as the Monarch finds a photo of he and Rusty playing in the front lawn of the Venture Compound as children with Monarch's parents and Jonas Venture in the background.

"Blood of the Father, Heart of Steel" has some small bits of SPHINX backstory but most of the SPHINX -related stuff revolves around Hunter and some OSI members reviving SPHINX after they feel OSI has become too weighed down by bureaucracy and policy.

Season 6, Episode 4: "Rapacity in Blue" Season 1, Episode 1: "Dia de los Dangerous!"

The previous season's "Rapacity In Blue" contains most of the backstory with the Monarch's father, who was the Blue Morpho. It doesn't contain much in the way of Blue Morpho's involvement with Jonas Venture beyond the first time we see the video Jonas uses to blackmail his friend. Much more of the story is revealed in this episode.

"Dia de los Dangerous!", the very first episode of the series, also contains the Monarch explaining his origin, which involves a plane crash in New Jersey that killed both of his parents. This is also why Vendata has flashbacks of a plane crash.

Season 3, Episode 11: "ORB"

"ORB" is still the only episode where Kano speaks. While searching for the ORB, Brock is lead to Kano's apartment. They briefly fight each other before Kano speaks what he thinks will be his last words. When Brock says that he thought Kano was a mute, Kano says that his silence was a choice. "I have taken from this world a great man. My silence is my penance." At the time (and for years after) everyone speculated Kano had killed Jonas Venture for some reason. However we now see that it was Venturion, previously the Blue Morpho, who Kano had to kill in order to save young Rusty Venture.

Season 1, Episode 11: "Past Tense"

In "Past Tense", way back in season one, Dr. Orpheus meets the original Team Venture, who mistake him for a foe and attack him. Action Man shoots Orpheus (in the arm) before Hank and Dean can explain that Orpheus is a friend, and in anger Orpheus takes Action Man's hands and tells him that he will die in "two years, seventeen days ... from a stroke". Assuming Dr. O's prediction was accurate, that is how much in world time has passed between this season one episode and the current time. Though since the end of the episode notes that Action Man is in a coma, it's also possible an even shorter period of time has gone by.

Season 1, Episode 6: "Tag Sale -- You're It!"

"Tag Sale -- You're It!" is the very first appearances of the shrink ray. Pete and Billy attempt to purchase it from Dr. Venture, but they start to bicker and Doc refuses to sell it to them. The shrink ray has appeared or been mentioned in a number of episodes since.