The gang stops at a nearby lake in the hopes of catching fish, but have no luck. However, Golem sees a beautiful mermaid named Undine, and he quickly falls in love with her. Undine says she likes him too, and makes him promise to protect her. Golem is happy to do so, and in exchange, she'll tell his friends about an island where they can catch as much fish as they want!
However, Undine then reveals herself as The Mole and backstabs the group! She has an edge on the heroes because Golem is protecting her from harm. But even though Undine uses powerful water attacks and tentacle arms, alone, she's no match for the combined Searchers.
As Undine orders Golem to attack his friends, she sets herself on fire when her tentacles make contact with the group's cooking oil and campfire. Undine is soon engulfed in a blaze, and Golem runs to help her despite what she'd done. Saying she doesn't deserve his friendship, Undine begs his forgiveness and leaps into the fire herself, dying. The group sadly watches as Golem tosses flowers into her lake.
This episode contains examples of:
- Always Save the Girl: Forced to choose between his friends and Undine, Golem decides to honor the promise he made to protect Undine, to the point that he's willing to leave the group in order to stay with her.
- Ambiguous Situation: Whether or not Undine was lying when she claimed that she can't leave her lake. Was that all part of her manipulating Golem? Or is that part of why she winds up throwing herself into the flames to free him from his promise — because she truly couldn't leave that area, and thus couldn't join them on their travels?
- Bitch in Sheep's Clothing: Undine initially presents herself as a kindly water nymph in order to hide her true intentions.
- Combat Tentacles: Undine's arms stretch in a way similar to a Jell's.
- Commonality Connection: Invoked by Undine, who appeals to Golem that she understands the depths of his loneliness, being trapped at her lake much like how he spent untold years guarding the grave of the discs alone.
- Downer Ending: The Searchers survive Undine's betrayal, and Golem's compassion gets through to Undine... but she still sacrifices herself, leaving him in shock and mourning her death.
- Family-Unfriendly Death: Undine burns alive in front of Golem. And she leaps into the flames willingly.
- Fighting Your Friend: After the others narrowly escape Undine's trap, Golem makes good on his promise and protects her. Undine tries to press this advantage and goad him into actively attacking the others.
- Good Eyes, Evil Eyes: When Undine reveals that she's one of the villains, she narrows her eyes.
- Heel–Face Turn: Undine, upon realizing that Golem truly cares for her.
- Heroic Sacrifice: After racing into the fire to save Undine, Golem turns away from the Searchers and starts to walk away, implicitly choosing to stay with Undine over continuing the journey. Undine decides to atone for her sins by willingly flinging herself into the fire, freeing him from his promise.
- Hoist by His Own Petard:
- Undine's own actions lead to her spilling the group's cooking oil on herself and into the campfire.
- In the uncut version, the Searchers are able to pull her out of the lake by grabbing one of her extended arms when she's attacking them.
- Kill It with Fire: The fire that Undine accidentally starts ultimately leads to her demise.
- Man on Fire: When Undine spills the oil, some of it splashes onto her arm, and some into the fire... leading to a spark that hits the oil on her arm and ignites it.
- Mega Maelstrom: Undine creates one in the middle of the lake when trying to drown the Searchers.
- The Mole: Undine pretended to be friendly, but used Golem to backstab the group.
- Nature Spirit: Undine.
- The Promise: Golem swears to protect Undine. Even after she reveals that she's working for Gray Wolf and attempts to kill everyone, he shields her from their attacks.
- Redemption Equals Death: Undine herself believes in this, and willingly consigns herself to the flames instead of letting Golem rescue her.
- Unreliable Narrator: It's unclear whether or not Undine was telling the truth when she tells Golem that she's unable to stray too far from her lake.
- Villainous Breakdown: When Undine sees how Golem was hurt protecting her, she starts lashing out wildly while yelling "They are my enemies; Get them!" This wild flailing is what causes her to spill the oil.
- Wingding Eyes: Suezo's eye gets a spiral as he's disoriented from being spun around in the whirlpool.
