1 Prof. Jones: Ah, Venice. My favourite city.
1 Monty: So, dad...
2 Monty: Now you've found the Grail, what are you going to do?
2 Prof. Jones: Oh, the Grail was just the beginning, Junior.
3 Prof. Jones: We must stop Hitler from getting the Spear of Longinus, the True Cross, the Emerald Codex of Hermes Trismegistos...
4 Monty: Why not just kill Hitler?
4 Prof. Jones: Where's the fun in that?

Alternate punchlines, thanks to Ral and margath. Insert as the dialogue in the final panel. I would have made graphics for these, but there's too many!

1. Monty: Why not steal Hitler's Brain in a jar?
   Prof. Jones: Don't be silly, that's just a fable.

2. Monty: Wouldn't it just be easier to do away with Hitler and his cronies?
   Prof. Jones: I don't know, Junior, that sounds awfully complicated.

3. Monty: Whoah there, Dad. I'm not contracted to do that many movies.

4. Monty: Dad, don't you ever think of taking a vacation?
   Prof. Jones: What? This isn't a vacation?

5. Monty: This is beginning to sound like a videogame, only you're not all that much to look at, Dad.

And yes, that's a photo of St Mark's Square in Venice, taken by yours truly. The full photo is here.