With him on Mercury are Helene deSilva, an attractive station commander with whom Jacob develops a relationship over the course of the book; Fagin; Pila Bubbacub, the library representative; his assistant Culla (a Pring); Dr. Dwayne Kepler (the head of the Sundiver expedition); Dr. Mildred Martine (a psychiatrist); and the exuberant journalist Peter LaRoque.

The Sundiver is a spherical ship designed to approach the Sun quite closely, with living quarters on one side and a large instrumentation section on the other.

When Culla realizes he has been discovered he retreats to the instrument side of the ship and begins disabling the equipment that propels the sunship so that it will fall into the photosphere, taking all evidence of his deception with it.

While Demwa and one of the crew attempt to disable Culla, Helene discovers that only the galactic technology has been sabotaged, and uses the refrigerator laser as a thruster to move the ship out of the sun.

Culla is killed, and the ship eventually escapes the sun, though all but Fagin temporarily "die" of hypothermia and frostbite from the refrigerator laser.

Although set in the same universe as the rest of the other Uplift books, it is set a considerable amount of time before the other books, and shares none of the same characters, apart from Jacob Demwa, who is mentioned as the mentor of Tom Orley and Gillian Baskin, and Helene Alvarez (née deSilva), who is mentioned in Startide Rising as Credeiki's former captain aboard the James Cook and who appears in The Uplift War to sign a treaty with the Thennanin.

The technique used to escape the sun alive when the Galactics' technology is sabotaged, using a refrigeration laser to dump the solar heat from the ship, is used again in Heaven's Reach.

Though Brin's human characters are rather two-dimensional and the story depends less on their interaction and development than on the setting and science, he is somewhat more competent in this area than [author James P. Hogan on Thrice Upon a Time].

"[1] Dave Langford reviewed Sundiver for White Dwarf #68, and stated that "Certain characters' weird actions are performed solely to help Brin's plot: but this is a first novel.