Short answers, pulled from the story.
An action-adventure game is a video game hybrid genre that combines core elements from both the action game and adventure game genres. These titles require many of the same physical skills as action games, yet they also offer a storyline, numerous characters, an inventory system, dialogue, and other features of adventure games.
Mark J.P. Wolf credits Adventure from 1980 for the Atari VCS as the earliest-known action-adventure game involving exploring a 2D environment, finding and using items which each have prescribed abilities, and fighting dragons in real-time like in an action game. Superman released by Atari in 1979 is cited by Brett Weiss as an early action-adventure game with Retro Gamer crediting it as the first to utilize multiple screens as playing area.
Games like Brain Breaker from 1985, Xanadu from 1985, Metroid from 1986 and Vampire Killer from 1986 further established the side-scrolling platform-adventure format. Over time this would evolve into the Metroidvania subgenre, which is a portmanteau of Metroid and Castlevania.
The Legend of Zelda Ocarina of Time from 1998 set a new standard for 3D action-adventure games. Its Z-targeting system solved the issue of 3D combat clarity while context-sensitive interactions and an expansive world made it a template for third-person adventure games.
Grand Theft Auto III released in 2001 combined the action-adventure template into a modern open-world sandbox allowing non-linear mission structures and systemic interaction in an urban environment. This title belongs to a subgenre of open world action-adventure video games in the third-person perspective where players may find and use a variety of vehicles and weapons while roaming freely in an open world setting.