The Doctor, River and others in “The Impossible Astronaut”

Be careful!

Careful. Tried that once. Ever so dull.

Shout if you get in trouble.

Don’t worry, I’m quite the screamer. [Climbs into the tunnel.] Now there's a spoiler for you!

So, what’s going on here?

Er... Nothing. She’s just a friend.

I think he's talking about the possible alien incursion.