The Doctor peels Vincent off himself and decides maybe Vincent needs some chamomile; outside, Amy is attacked by something and goes screaming. The Doctor, very scared, runs out and finds her having been hit from behind by something or another. Just as they're getting her set to rights, Vincent starts fighting something invisible. The Doctor and Amy -- which is odd, because they totally know there's an invisible monster roaming the streets, and they are here to find a giant rooster monster, and they know it's connected to Vincent -- are very social-workery about Vincent's invisible-monster fighting, like, "He's having some kind of fit" and "I'll try to calm him down" are some things they say. Rather than, say, "Guess there's an invisible monster. How intriguing."
The invisible monster smacks the Doctor with its giant tail, and then the Doctor helps Vincent fight the monster, which means generally that the Doctor gets knocked around several times while cutely swinging a stick at nothing in particular. Vincent chases the monster away, and the Doctor continues fighting windmills for a bit, and then back inside Vincent paints over a very famous canvas of some kind -- Amy nearly hurls with horror -- so he can sketch out exactly what an invisible rooster monster looks like. It's quite detailed. The Doctor's like, "Well, hell."
The Doctor heads over to the TARDIS to find some kind of technology, after distracting Amy and Vincent with a variety of cute facial expressions and his particular brand of sexy silliness turned up to about a hundred. The thing invisibly chases him inside and he finally finds the device, which is like an iPad made of bicycle parts that identifies things you put in front of it. Like he sticks out his tongue at it, and it starts throwing up pics of him from One to Two and Three. He shows the device -- "an embarrassing present from a dull Godmother with two heads and bad breath" -- Vincent's sketch, but it doesn't work. "This is the problem with the Impressionists! Not accurate enough."
Outside the TARDIS, heading back through town, the creature looms behind the Doctor, showing up on the mirror-screen, and it identifies the monster for him. Of course he doesn't know it's behind him, because huge invisible roosters are super stealthy, especially when -- spoiler -- they are blind. "You poor thing," he says, "You brutal, murderous, abandoned thing. I hope we meet again soon, so I can take you home." Short chase, of course, immediately follows.