The radiation storm overloads the TARDIS's systems, leaving only minimal power. They land on the nearest planet to make repairs.
Rose makes tea over a lab burner. The Doctor and Jack gobble some sandwiches, then return to work. "Got to check every system, make sure they're in good workin' order," the Doctor explains.
It takes two days. Rose luxuriates in a hot bath. The blokes collapse on Jack's bed; even the Doctor needs sleep. He wakes to find Jack's hand stroking a very sensitive spot.
Jack's smile promises even more. "Gotta check that every system is in good working order."