The first day of testimony in Hunter Biden’s trial on gun-related charges kicked off Tuesday with the surreal sound of the defendant’s own voice ringing through the courtroom, narrating his descent into drug addiction, when prosecutors played the audiobook of his memoir.

It ended with bitter written words: expletive-laced, panicked texts to Hallie Biden, his brother’s widow and his onetime girlfriend, berating her for disposing of a handgun and warning, presciently, that it might set off a federal investigation.

The government’s case against President Biden’s son — for all the drama, media swirl and complex political dynamics — is pretty straightforward legally: proving that Mr. Biden was abusing drugs when he filled out a federal firearms application claiming he was not an “unlawful user” of controlled substances.

Prosecutors stressed that point in their 15-minute opening argument before a packed courtroom that included Jill Biden, the first lady. Lying on a federal gun application is illegal and “nobody is allowed to lie, not even Hunter Biden,” said Derek Hines, a top deputy to the special counsel, David C. Weiss.

“Addiction may not be a choice, but lying and buying a gun is a choice,” Mr. Hines said.

“Nobody is above the law,” he added, echoing language the Justice Department has repeatedly used to justify its prosecutions of former President Donald J. Trump.

Almost all the events covered in the trial happened in 2018, when Joseph R. Biden Jr. was out of office.