Crystal Venom is the ninth episode of Season One.
Allura considers making a heartbreaking sacrifice to save the team when the Castle mysteriously turns on them and sends them hurtling toward doom.
Princess Allura is still weakened from the rejuvenation ceremony she performed to save the Balmera, but is spending her time enjoying the company of her father's artificial intelligence in the memory chamber of the Castle of Lions instead of resting. While speaking to King Alfor's AI, she reflects on her memories of Altea and her family and admits how deeply she misses them, wishing things did not turn out the way they have. King Alfor's AI is understanding of her pain, but reminds her that leaders must do what is right, even if it means great sacrifice. Coran finds Allura in the Chamber and scolds her for not resting. As the Royal Advisor helps her to her room, Alfor's AI promises that he will be waiting for Allura's return. The hologram of Altea vanishes, Alfor's own image flickering before it disappears.
After taking Allura to her room, Coran calls for Pidge, who is studying Sendak's Galra Crystal, to meet the other Paladins the detainment room. She is annoyed at being interrupted from her work but complies, leaving the ominous glowing Crystal behind. The Paladins gather around the sleep pod holding Sendak as Coran hooks up a memory extraction system to the sleep pod. Coran is hesitant about using the technology on an unwilling participant who is not Altean, since the technology is not designed for such a thing, but Shiro is hopeful they can extract information about the Galra Empire and its military from Sendak's memories.
The Paladins and Coran wait an extended, boring stretch of time and witness no memories being extracted by the storage system. Keith is the first to leave to train at the training deck, followed by Pidge seeking to continue studying the Galra Crystal. Hunk finally leaves for the kitchen and this prompts Coran to admit he cannot wait around any longer either: the Castle of Lions has taken heavy damage and exhausted its systems, all of which Coran must deal with. Shiro is adamant about staying to monitor Sendak.
Lance tries to leave simply to relax elsewhere, but Coran insists the Blue Paladin will be helping him with the Castleship instead because he is the last Paladin there, sans Shiro, and Lance's choice of activity does not seem important. Lance grudgingly complies and leaves with Coran, but not before he asks Shiro to inform them of any developments; Shiro's reply is delayed and distracted.
The Castle Attacks
While the team waits for Sendak's memories to be extracted, the Castle of Lions begins displaying strange behavior. First to suffer is Hunk, who simply tries to get a plate of food goo from the goo dispenser and finds it not working. As Hunk inspects the machine, he gets attacked by a blast of goo and becomes pinned down behind a counter top under heavy goo fire. Pidge happens across the chaos in the kitchen and comes to Hunk's aid; they use plates as shields to flank the food goo dispenser and cease its firing. The two Paladins chalk the incident up to a simple malfunction and leave the gooey mess for Coran to deal with.
In the sleep chamber, Lance complains about having to clean the cryo-replenishing pods with Coran, who begins reminiscing about his days as a young Altean cadet cleaning pods. While Coran is distracted by rambling about his past, Lance enters a pod to clean it, only for the pod to encase him and descend into the floor. By the time Coran turns around, Lance is nowhere in sight. The Royal Advisor is nonchalant, thinking Lance is being a typical Paladin and shirked his duties.
At the training deck, Keith is sparring against the Gladiator robot using his Bayard. Having finished battling once, he orders the training system to engage the next highest level. The Red Paladin battles against the newly dispensed robot but the fight is too intense for Keith to keep up, so he commands the training system to disengage. Strangely, the system does not respond to any of Keith's orders; the Gladiator robot turns rogue and rushes Keith, knocking the Red Bayard from his hands and sending the Paladin flying across the floor. Keith manages to use his agility to dodge the robot's next strike and recover his Bayard, then flees from the training deck to the safety of the hallway. To Keith's terror, the Gladiator robot forces itself into the halls to pursue him.
Shiro remains completely unaware of the chaos plaguing the Castle while he speaks to Sendak in the quiet detainment room, ordering him to give them information. There is no response from the sleeping Galra commander nor the memory storage system. In frustration, Shiro slams his fist on Sendak's pod and insults him as a broken soldier. Apparently in response, a memory finally enters the storage system, leading Shiro to believe that Sendak can actually hear his words.
Coran is finishing cleaning pods in the sleep chamber and reaches the pod that Lance had become trapped in. The sight of the frozen Blue Paladin startles the Royal Advisor, but he is dismissive of the shivering Paladin's claims that the pod automatically locked and began cryogenically freezing him, thinking either Lance does not want to admit to falling in, or that the pods are simply malfunctioning. Lance suggests the Castle is haunted and trying to kill him; Coran admits a spaceship embodying supernatural technology that science alone cannot explain would seem "haunted" to primitive Earthlings, but retains that nothing unusual has happened.
After Coran leaves to handle other duties, Lance sits alone in the sleep chamber struggling to get warm, and hears rumbling of the Castleship that only fuels his fears. The pod he had been trapped in suddenly descends into the floor, causing Lance to scream and flee the chamber in terror.
Elsewhere in the Castle of Lions, Allura is awakened by the voice of her father, King Alfor, as his holographic AI has appeared in her bedroom. Strangely, as Lance nervously scurries through the halls of the Castleship, the lights around him go dark, and he too sees an image of King Alfor's AI - although it flickers and he mistakes the disappearing image for Coran. When Lance calls out to the Royal Advisor, he hears Coran reply, calling for help and claiming that he is trapped in the airlock leading to the void of space. In reality, the voice is one created by the ship itself. Lance rushes into the airlock to assist, but finds no one inside, and the door locks behind him. The Blue Paladin laughs it off as his team playing a prank; unfortunately, the alarms indicating the airlock will open to space in 30 ticks prove otherwise. No one is around to hear Lance's panicked cries for help.
In the detainment room, Shiro begins interrogating Sendak for information. When he asks about Zarkon's greatest weakness, he suddenly hears Sendak's voice replying to him, taunting the Paladin about the zero possibility of being able to defeat the Galra Emperor. However, Sendak remains unmoving and asleep as the words are heard to Shiro's ears. The Black Paladin tries ignoring the incident and asks where Sendak would strike an attack on the Galra Empire, but hears the commander's voice again: words that tempt Shiro to join Zarkon instead of fighting against him. Shiro's mind is ensnared by the supernatural occurrence and his body frozen in fear as he sees a flash of Sendak's face clearly awake, although the commander is still asleep in his pod.
Shiro continues to monitor Sendak and begins interrogating him, but is mentally tormented by Sendak's voice prompting him to eject Sendak into space. Coran concludes that Alfor's A.I. has been corrupted by the lingering effects of the Galran crystal as they try to stop Allura, convinced that Alfor is taking her back to Altea, from driving the Castle-ship into an imploding star.
At the urging of the real Alfor, Allura tearfully destroys her father's memories in order to save the ship. Despite the loss, Allura stands strong and reassures the others that is her father's legacy is Voltron.