Okay, so the Brig and Sarah are in Bessie, which Benton is driving, the Doctor's hanging out the side, chasing loon of the week who has stolen the Whomobile, then he jumps out and gets in a flying contraption; the Brig makes Benton let him drive, that's when the police spot them and starts chasing them, they stop, shout at each other, do a Chinese fire drill, and the thief takes the flying contraption, Sarah and Doctor get into the Whomobile, which can now apparently fly, and Benton and the Brig follow on ground in Bessie.
And trailing after them is a very confused constable.

And now they're on speedboats. Correction, one speedboat, one hovercraft.