When a dragon that causes man-eating whirlpools threatens Berk's trade ships, the Dragon Riders must remove it without getting caught in its vortex.
- Androcles' Lion: The Submaripper is hostile to Hiccup because humans chained it to its current spot, but saves Hiccup's life when he breaks the chains.
- Berk is low on both funds and resources thanks to the loss of all its gold, and the loss of various trade ships.
- Hiccup makes a diving mask out of Deathsong amber, and later a diving bell too.
- Drowning Pit: The diving bell ultimately becomes this for Hiccup when its chain breaks and it sinks to the bottom with Hiccup trapped inside. Toothless is unable to get him out, though ultimately the Submaripper saves him.
- Hostage Situation: Ryker has a boat towed by Seashockers, knowing the riders are too noble to harm the dragons to get him and his brother.
- I Owe You My Life: After Hiccup frees the Submaripper, it returns the favor by pulling him and his diving bell out of the water and onto the boat.
- I'm a Humanitarian: Both literally and figuratively; the Submaripper is introduced to the audience by eating three trading ships, and to the riders while trying to eat a Scauldron. Albeit the attacks are indicated to be more about it being territorial and defensive rather than hungry.
- Know When To Fold Them: Ryker, unlike Viggo, calls for a retreat when the Submaripper is loose and the riders are bearing down on them.
- This Is Unforgivable!: While Viggo has always been an enemy, his willingness to target innocent civilians in retribution for Hiccup's raids on his outposts is treated as beyond the pale.
- Together in Death: Toothless plans to pull this when it looks like he will be unable to save Hiccup from drowning in the diving bell, choosing to stay with him and drown as well rather than leave Hiccup on his own. Luckily, they are saved by the Submaripper.
- Villain: Exit, Stage Left: Ryker and Viggo escape on Seashocker pulled boats after the Submaripper is freed.
- Villainous Breakdown: When defeated, Viggo loses his cool for once and demands the hunters stay and fight, requiring Ryker to be the voice of reason and help them retreat.