The official explanation was suicide. But Ben Du Toit, an "ordinary, . . . unremarkable" Afrikaner, had a feeling that Gordon Ngubane, his black fellow worker and acquaintance of 15 years, had not really killed himself while in the custody of the security police.

Du Toit's odyssey to discover the truth about Ngubane's death shatters the assumptions of his Afrikaner world view as well as his own life, and is the theme for "A Dry, White Season," the most recent novel of dissient, Afrikaans-language author Andre Brink.

It is Brink's most audacious criticism so far of his racially obsessed people, the 2 1/2 million whites of Dutch descent who rule this country. Brink pinched an especially sensitive official nerve focusing on what Afrikaner author Jan Rabie called "the inherently fanatical self-protection" of the security police, who brought a wave of international opprobrium to South Africa when black consciousness leader Steve Biko died in their custody in 1977.

As a result, "A Dry White Season" became the first Afrikaans novel banned even before it reached bookstores. The official reason: It is "calculated to undermine the status of the South African security police . . . [and thus the] ability to ensure state security." The head of the security police announced that he was studying the book to see whether Brink should be prosecuted for defaming his force.

The outcry was such, however, that the censorship appeal board overruled the banning decision on the theory that the book is its own worst enemy. Calling Brink a "malicious writer," the chairman of the appeal board, J. C. Van Rooyen, said. "The one-sidedness of the attack" on the police, the "shallowness in characterization" and the "stereotyped descriptions" would discredit the novel with readers.

"The basic story line is so vital to what is happening in South Africa today," said the rebellious author during an interview in his ivy-covered, century-old home, where he lives with his wife and four children. He teaches Afrikaans at nearby Rhodes University.

Blue-jeaned and bespectacled, Brink relaxed in an easy chair in his book-lined study, occasionally running his hand through a shock of curly, sandy hair. His relaxed demeanor contrasts with the Old-World formality of his afrikaans-accented English and with the intensity of his feelings about "A, Dry White Season."

"I wanted to make sure no one could say afterwards, 'I didn't know' -- the old sort of Nazi excuse -- about the intolerably inhumane way the blacks are treated as a whole," he said.

The story "jelled" in his mind in 1976 after the death of Mapetla Mohapi, a young black activist and friend of Biko. When Biko died in September 1977 Brink had to stop work on the novel for a year because it seemed "terrible to dabble in letters when real people were really dying . . . I had to get my composure inside before beginning again.

"The theme is a metaphor of what I really tried to explain, which is the isolation of people apartheld brings about. Whiteness and blackness creates a gulf as a result of apartheld. It's impossible to know people; we become crucified to color," Brink said.

In the book, Du Toit's life begins to entwine fatally with his black colleague's when Ngubane, the black janitor at the school where Du Toit teaches, relates what he has heard about his missing son, Jonathan. A friend told Ngubane that after he and Jonathan were detained by police, Jonathan was interrogated for 20 hours and made to stand on blocks with a weight tied to his genitals. He was taken to a cell, screams were heard and Jonathan was never seen again. The police told Ngubane his son was shot during a riot in Soweto and buried. When the janitor persists in his inquiries, he also is arrested.

The news of Ngubane's suicide comes like a nightmare to Du Toit. At first, believing as most Afrikaners do in the rectitude of the white authorities, he thinks the suicide version is a bureaucratic error he can have adjusted easily.

After investigating the death, Du Toit concludes that Ngubane was tortured and beaten to death by the police. Rather than a mistake, he finds coverup at every level in his community. Even his church minister participates.

Du Toit's quest costs him his job and the friendship of his faculty peers who do not approve of his interest in the fate of black detainees. His wife leaves him; even his daughter betrays him by telling the police where he has hidden the evidence he has gathered about Ngubane's death.

In a final humiliating incident, Du Toit is attacked by a mob of blacks outside the Soweto home of his taxi driver friend -- scorned by the victims of the unjust system he is fighting.

He finally realizes that the police will stop at nothing to prevent the truth from coming out, and in anticipation of this, he sends his notes to a journalist friend, who narrates "A Dry, White Season." After mailing them, Du Toit is killed by a hit-and-run driver.

Part of the sensitively of the authorities to the book stems from the fact that this devastating moral critique of Afrikaner-dominated society is given in their own language -- usually revered as a vehicle for Afrikaner natinalism.

Afrikaner literary critic and univeristy professor Ampie Coetzee believes that his people's defense of their language also creates problems for them. "Afrikaans is glorified. It's a holy language. Our literature is holy . . . and they want to protect it. They are very proud of it so [the authorities] can't go too much against the writers, even though they're saying things they don't like," he explained.

Brink's own dissidence stems in part from love of his language. "I want to show that the Afrikaans language and culture also have a disident vein, that it is strong and more virile than just the language of the oppressor," he once told a local paper.

With "A Dry White Season," Brink resorted to what he calls "guerrilla warfare" with the censors, mailing out 3,000 copies through a book club called "Taurus" organized by dissident Afrikaner writers for the purpose of helping controversial authors reach at least a preliminary audience before the censors can act. CAPTION: Picture, ANDRE BRINK . . . labeled "malicious" by censors