The gist of the plotline is that Hiccup is flying around and stumbles upon a dragon trapping band. Then, a bunch of dragons take him and Toothless to a frozen spiky kind of a dragon cliff and he meets an interesting dragon "master". It turns out it is his mother and she's been living in the cliff with the alpha dragon there for 20 years straight.
Thankfully, there are still the baby dragons, which will not be controled by anyone. Not alpha and not really by a human either.
So they get to Berk and Hiccup regains control over Toothless - through their friendship.
After that, Toothless challenges the "evil" alpha and wins, thus kinda becoming the new alpha for all the present dragons.
After this, I did a bit of a research and found out there will be a third movie, which I just can't wait to see, seeing as Hiccup become the village leader and I want to see how that will turn out. I'm also interested in what new tricks on dragons there will be.
Okay, but seriously, if you're into animated movies, I think you should watch this one. It will blow you away!
Enough said. I'm now gonna go and rewatch the first Iron Man movie.