Thor became the hostage of some mysterious enemy forces. He didn't succeed in finding the Infinity Stones he though he wanders towards a skeleton sharing his cage. The conclusion he comes to is saying that in order to get the answer from Surtur, you have to be captured sometimes. After he frees himself from the cage, he arrives to Surturs world where he has to find out his absence leaves Asgard vulnerable as Odin is not there any longer. Also he tells Thor that Ragnarok is about to come to Asgard and there's nothing he can do to prevent him, though meanwhile he reveals the secret to Thor that his crown is the source of his powers.
Even for a Marvel film, Thor: Ragnarok is a weightless lark, almost to a fault. Waititi has so much fun playing with his toys that, at times, he forgets to offer the proceedings enough import. These are marginal concerns, at the end of the day.
In its own weird little way, Thor: Ragnarok manages to poke fun at the constant churn of myth and entertainment of which the movie itself is a part. It's a candy-colored cage of delights, but it is a cage nevertheless - and it doesn't hide that fact.
Granted, there are probably more monsters and CGI battles and explosions than the movie needs. But director Waititi and star Hemsworth keep this Thor party hopping like it's 1999 and Ragnarok will never come. Good times.
Thor: Ragnarok is packed tight with zooming space vehicles and noisy thunder battles, but the movie's extravagant excess is more narcotizing than energizing. Even poor Thor seems lost in all of it, and he's supposed to be its star.