Fortune is on the crews' side - the transporter on the ship is still working, but needs someone to manually operate it. Marvin is therefore pressed into volunteering to sacrifice himself so the rest of the group can escape. Arthur and Ford get separated from Zaphod and Trillian and find themselves on board a space ship about to crashland into the prehistoric past of a planet that the two of them find strangely familiar. If this is indeed Earth, then history as they know it is about to be changed - and so is the Ultimate Question to Life, the Universe and Everything....