Bubble in the Bathtub - Jo Nesbo

The Fart Powder was such a successful invention that Doctor Proctor, Nilly, and Lisa couldn't stop there. Next up: a time-travelling bathtub. You just hop in, lather up the Time Soap, and wish for where you'd like to go. Doctor Proctor has plans for this new invention. You see, he lost his true love years ago, when Juliette Margarine married an evil count. The good Doctor has never quite gotten over this, and he's going back to change it. But when things go wrong, it's up to Nilly and Lisa to travel back in time to right all wrongs and reunite the two lovebirds. Nothing is quite so simple in a Jo Nesbo book. Enter a herd of hippos, a scheming assistant, and Time Soap that keeps going awry, sending Nilly and Lisa to the storming of the Bastille! Fortunately, as in every Jo Nesbo book, the Fart Powder solves everything.