This is my effort at figuring out How To Train Your Dragon. It actually has very few negative reviews which makes this a bit trickier, but I think I got a passable summary. Obviously, spoilers are ahead.
...two fine battle scenes... bookend the movie
Hiccup is a lonely, scrawny Viking lad, more interested in weird inventions than the local pastime: killing dragons.
During a dragon raid (on the island of "Berk"), he lucks out and shoots down the most dangerous flame-tongued flying beast of them all -- the Night Fury. Sounds like a kind of cologne popular at proms.
When he does finally man up, he downs an elusive Night Fury dragon (later nicknamed Toothless). But instead of finishing the job, Hiccup ends up befriending and then, you guessed it, training his dragon.
This monster turns out to be a big friendly pet, a sort of bat-Labrador-kitty. He likes being scratched on the neck, he purrs, and his reaction when rolling around in a catnip-like grass is pure Cheech and Chong.
Gradually, Hiccup starts to learn the dragons' ways, though he must keep this knowledge secret from the town where they're the enemy. Hiccup also must participate in a dragon-slaying class, though he seems likely to apply for conscientious objector status.
[Hiccup is] hoping to impress his father (Gerard Butler), a beefy Norseman with a Glasgow accent and triceps like tree trunks.
Hiccup's gruff, he-man father, Stoick the Vast (Gerard Butler), wants his son to be tougher.
Father and son, though inevitably scheduled for reciprocal self-actualization (brain, say hello to brawn, and vice versa), spend much of the movie at loggerheads because junior would rather fly around on, instead of slay, his newfound scaly friend, whose cute, big poonim bears an incongruous resemblance to the critter from Lilo & Stitch. Intentionally or not, all of the dragons are built more for stand-up comedy than for terror.
There's a hanger-on who seems to have no role in anything except wisecrack delivery (he's played by Craig Ferguson, who like Butler lets his Scotch accent go wild; oddly, everyone else sounds American) and an adolescent girl (America Ferrera) who is, naturally, tougher and feistier than any boy.
Also, there's another wisenheimer in Hiccup's training class who is drawn to look exactly like Jack Black, though Black's price must have been too high because the voice is Jonah Hill's.
Hiccup begins to think about a different approach to dragon-human relations.
Shouldn't the dragon wars stop? Shouldn't we all live together in a warm, friendly human-dragon commune? Hiccup tells the dragons, "Everything we know about you guys is wrong" and believes the beasts are not killers -- "They defend themselves, that's all." Of his own folk, he says, "The food that grows here is tough and tasteless -- the people, even more so."
...the wan love that blossoms between Hiccup and a feisty young Vikingette voiced by America Ferrera. Better is some funny business when fledgling killers-in-training meet baby dragons-in-training, supervised by the deliciously hectoring voice of Craig Ferguson.
The script is pretty straightforward. Hiccup learns how to subdue dragons without killing them, and after a spat with the leader of the tribe (also Hiccup's father), they all learn tolerance of dragons, and Hiccup gets the girl. Spoiler for every movie ever, by the way.
The last, biggest problem -- and I'm going to ruin the climax a bit -- is that the theme of "we don't have to kill dragons" is kind of undercut by the eventual solution to everyone's problems, which is, you know, killing a gigantic dragon. I guess tolerance goes to the wayside when the subject is super-huge. Regardless, it's a frustrating example of how the movie wants to have it both ways.