When Dr. Seward gets to Renfield's room, he finds the patient lying in a puddle of blood. Renfield looks like his face got smashed up and his back is broken.
Dr. Seward sends for Van Helsing, who comes at once, bringing his surgical case.
Renfield's skull is fractured, so they want to "trephine" (i.e., to drill a hole in the skull) to relieve the pressure. (Seriously, this was a state-of-the-art, high-tech medical practice at the time.)
When Renfield regains consciousness, he tells them that Dracula did it.
He says that Dracula had come to the window in the form of a mist two days before, and had asked to be let in.
Dracula then summoned rats and flies for Renfield and made the dogs howl in the distance.
Renfield invited him in.
Then the next day, Dracula came back and let himself in without being invited (apparently it only takes one invitation).
Renfield was mad about it. When he saw Mina that day, she looked bloodless to him.
Then, today, when Dracula tried to come in again, Renfield tried to stop him.
But Dracula smashed him onto the ground, which is how he got his injuries.
Van Helsing and Dr. Seward now know what Dracula is after: Mina.
They go get their anti-vampire weapons and head to Mina and Jonathan's room.
They break down the door.
They find Jonathan unconscious on the bed, and Mina sitting up on the edge of the bed.
Dracula is standing next to her, holding her wrists in one hand and with the other, forcing her to drink his blood from a cut in his chest.
Van Helsing and the others move forward, holding the sacred communion wafers and their crucifixes.
Dracula disappears as a mist, and Mina screams.
They wake Jonathan up from his vampire-induced trance and tell him what's going on.
Mina tells them everything: how she's had weird dreams lately and has felt weak.
Oddly enough, when Dracula was actually touching her, she didn't want to stop him from drinking her blood—must be something about his powers.
And after making her drink his blood, Dracula told her that she was in his power, and that even from far away he'd have the power to summon her and make her come to him.