Sword Art Online

Sword Art Online (often abbreviated as SAO) is a popular isekai/action anime based on Reki Kawahara's light novels.

Here's a quick, non-spoiler-heavy synopsis of the core premise and first season (which most people refer to when asking about "the anime"):

In the near future (2022), a revolutionary virtual reality MMORPG called Sword Art Online launches, using a helmet device called NerveGear that fully immerses players in the game world by connecting directly to their brains.

On launch day, 10,000 players log in—including the protagonist Kazuto "Kirito" Kirigaya, a skilled beta tester who prefers playing solo.

But right after they enter the game's massive floating castle world called Aincrad, the logout button disappears. The game's creator appears and reveals a horrifying truth: players are now trapped inside. If they die in the game, they die in real life. The only way out is to clear all 100 floors of Aincrad and defeat the final boss.

The story follows Kirito as he fights to survive and escape, forming key relationships (especially with the determined warrior Asuna Yuuki), while dealing with the psychological toll, player conflicts, guilds, bosses, and the harsh reality that every battle could be their last.

The first season (Aincrad arc + a follow-up arc) covers this death-game scenario and its immediate aftermath, blending intense action, romance, and themes about virtual worlds blurring with reality.

Later seasons dive into new VR games and threats (like Gun Gale Online, Alicization, etc.), but the classic "SAO" hook is that original trapped-in-a-death-game story.

It's a very influential series in modern anime—love it or criticize it, it basically helped popularize the trapped-in-a-game trope! If you're just starting, seasons 1 and maybe Alicization are the most commonly recommended.