In his headiest days as a corporate titan, Sumner Redstone had his hands in seemingly everything. Nary a story or new program launched in his sprawling media empire — reaching from Simon & Schuster and the CW network to Paramount Pictures and cable outlets like MTV to Nickelodeon — without the wily Bostonian having a say.

Redstone believed so deeply in his own powers that he told more than one interviewer he did not need a corporate succession plan. He said he would never die. Alternatively, he claimed that his plan for the future of his two media conglomerates — CBS Corp. and Viacom — was so airtight it would give him control even from the grave.

Musings on immortality feel increasingly fleeting for the 92-year-old magnate, who now requires around-the-clock care at his mansion in Beverly Park and often needs the help of a nurse or speech therapist to make himself understood.

Depending on what happens in a Los Angeles courtroom in a trial that begins Friday, Redstone could lose even more of his prized autonomy. A one-time lover and long-time companion, Manuela Herzer, will begin to mount a legal case to have Redstone declared mentally incapacitated and to have his care put in the hands of others.

The tentacles of the five-month-old case have stretched to raise questions not just about Redstone’s care, but about the succession plan for Viacom and CBS and about whether the magnate’s corporate bosses hid his true condition from the public and shareholders. Impressions left in what is expected to be a five-day trial could impact many, particularly Viacom CEO Philippe Dauman and Redstone’s daughter, Shari, who is a board member of both corporations and who has harbored ambitions to succeed her father.

After opening statements in the case Friday morning, it is expected that the downtown courtroom will be closed to all but the parties involved in the dispute for what could be the most dramatic moment of the trial. Superior Court Judge David J. Cowan will watch a videotaped deposition of Redstone. It will be the first time since the fight over his care began just before Thanksgiving that the billionaire will have a chance to make clear whether he remains capable of taking care of himself.

Cowan ruled earlier this week that the video will be shown only in his closed court, in order to preserve a measure of dignity for the frail nonagenarian. The media and public will be given a transcript of Redstone’s testimony, limited to a half hour, only afterward.

Lawyers for Herzer, 52, pushed hard to have Redstone testify, either in person or in a deposition. They contend that it is obvious that he can no longer care for himself.

Herzer filed her case after being thrown out of Redstone’s home in October by members of his staff and one of his estate lawyers. Two days later, Redstone’s team filed a new document, removing Herzer as Redstone’s health care agent — the person empowered to make medical decisions in the event that an individual becomes mentally incapacitated.

Herzer said in her lawsuit that unnamed individuals filed the new advance health care directive without Redstone’s approval and that the mogul had been determined that Herzer oversee his care. Herzer, a socialite of Argentinian descent, said she intervened only out of love for Redstone and concern he was not getting adequate care.

Redstone’s lawyers countered that it was money, not love, that made Herzer so determined to be allowed back into his life. They said Herzer already has received $70 million in cash and other assets from Redstone since 2009. And she had stood to get another $70 million — $50 million in cash and Redstone’s $20 million home — until he threw her out of the house and changed his estate plan.

Redstone’s lawyers argued that, in attempting to invalidate Redstone’s health care directive, Herzer really was angling to overturn the alterations that cut her out of his estate.

The trial will, in essence, be a contest between Redstone’s two “families” — one led by daughter Shari and his other blood relatives and the other headed by Herzer and including one-time lover, Sydney Holland, who was also ejected last year from Redstone’s home.

Shari Redstone and her children claim that Herzer and Holland inserted themselves into Redstone’s life and did everything they could to remain there, while obstructing efforts by the magnate’s family members to communicate with him.

Herzer contends that Redstone’s family was inattentive and disinterested. She has said that he effectively chose an alternative family that loved him. “You don’t pick your family in your life, but Sumner Redstone does,” Herzer told Vanity Fair last year.

She also told the magazine that she would be stunned if Redstone cut her out of his will, because he “said to me a million times, ‘Manuela, what’s mine is yours.’” She also pledged in the magazine article that she would not challenge Redstone’s dictates, unless someone else tried to intervene first. According to her legal filings, Redstone made her “the one person in the whole world whom he trusted to be his health care agent.”

The case has been aggressively contested since the start. After Redstone’s staff got Herzer to leave the Redstone mansion, she fought to return to get some of her personal things. Her lawyers threatened to invite the gossip website TMZ along and to file a “multi-million dollar lawsuit” if anyone disturbed her possessions.

Herzer’s November lawsuit painted Redstone as living a sad and tawdry twilight life. The complaint described him as a “living ghost,” who could scarcely communicate but still made it clear some of his appetites had not abated. He daily demanded servings of steak and sex from a much younger woman, who would be summoned to his home, Herzer claimed. He was so far gone, though, that a nurse would “direct … the intimate actions” between Redstone and his partner.

Similarly, Redstone began to demand to eat steak daily, Herzer claimed. Even when advised that it would be difficult for him to consume, the billionaire kept demanding steak and usually got it.

Herzer’s lawyers said they needed to give details of Redstone’s troubled existence to show how much his mental faculties had declined. The magnate’s legal team countered that all of Herzer’s intimate disclosures revealed how little the one-time companion really cared about Redstone’s reputation and well-being.

The two sides neared a settlement agreement last month, but an 11th hour disagreement scuttled the deal, clearing the way for the trial.