This is the one where all the Vikings of Berk are slaves to Alvin the Treacherous, and they have to search for the Dragon Jewel like doomed cockle pickers. We finally find out what happened to Fishlegs when he was a baby, and he discovers that a Deadly Shadow dragon will protect him come what may. And Hiccup relationship with his mother. In the earlier books she has been 'absent, questing', whereas now things take a turn for the worse when she kidnaps him, and deliver him to his worst enemies. But in the end, she has faith in him, and persuades the nicer half of the vikings to take the slavemark as the dragonmark and return to his side.