Voldemort basically rode Professor Quirrell back to Hogwarts.
But Professor Quirrell wasn't strong enough to help Voldemort, and he had to hide out as a spirit once again.
Then, Wormtail came looking for Voldemort less than a year before.
Wormtail had been in hiding for over a decade, but after being discovered, he came looking for Voldemort out of desperation.
With instructions from Voldemort, Wormtail built Voldemort his temporary, weak body.
Wormtail also brought a nosy witch from the Ministry, Bertha Jorkins, to Voldemort.
It was from Bertha Jorkins that Voldemort found out about the Triwizard Tournament at Hogwarts.
That's when Voldemort first began to plan to capture Harry Potter so he could perform a spell that would bring back his body permanently.
As luck would have it, Voldemort's one faithful servant – not Wormtail – managed to get himself stationed at Hogwarts. (Who? Who?)
This servant put Harry's name in the Goblet of Fire.
This whole tournament has been leading up to bringing Harry to the Riddle grave to perform the spell we just saw, bringing Voldemort back to life.
Now that Voldemort has been brought back to life using Harry's blood, Harry's mother's protection no longer works.
So Voldemort can hurt Harry. And he does, by casting the Cruciatus Curse on him right then and there.
Harry is in agony.
And Voldemort has decided to prove his power "by killing [Harry], here and now, in front of [the Death Eaters], when there is no Dumbledore to help him, and no mother to die for him" (33.106).