The series follows the adventures of Will, an orphan, who is chosen as an apprentice Ranger, one of the skilled trackers, archers, and warriors in the service of the King of Araluen.

Soon after, Halt and Will leave for the Ranger Gathering, where they receive a report that the Kalkara, vicious creatures under the control of Morgarath, have reappeared in Araluen.

Finding that Halt is battling the Kalkara alone, Sir Rodney and Baron Arald manage to knock one of the beasts into the fire, but are badly injured by the other.

He dispatches an auxiliary force, led by Halt, to stop another group of Skandian warriors that have come up from the coast to outflank the king's army.

This is done successfully, and Halt and his troops disguise themselves as the Skandians, tricking Morgarath into ordering the Wargals to charge into the open plain.

His army destroyed, Morgarath offers a flag of truce and prepares to challenge Halt to single combat to avenge his own defeat.

Erak, the captain of the Skandian crew that captured them, helps Cassandra escape with Will, and the two take refuge in a small log cabin for the winter.

Meanwhile, Horace and Halt set off to rescue Will and Cassandra by crossing into the neighbouring country of Gallica and making their way north along the coast.

In Oakleaf Bearers, called The Battle for Skandia in the United States, Cassandra is captured by a Temujai warrior while scavenging for food.

Will's friend and former wardmate Alyss, also in disguise, is sent to help him, but is captured by the rogue knight Keren who is responsible for poisoning Lord Syron.

Together, they devise a plan to take control of the castle and rescue Alyss, with the help of a group of Skandian raiders who have been stranded in Araluen.

They infiltrate the castle and Will reaches the tower where Alyss is held, but Keren hypnotises her to believe Will is the enemy and orders her to kill him.

Because Erak's first mate Svengal believes that he was betrayed by a rival Skandian named Toshak, the raiding party goes to Araluen instead of Skandia for the ransom money.

Princess Cassandra returns with Svengal and the rest of Erak's crew to Arrida in her father's place to negotiate prices with Selethen, the leader of Al Shabah.

After the negotiation is over, Selethen reveals that Erak was being held in Mararoc, a fort in the desert, but as they are traveling there, Will loses his horse Tug during a sandstorm.

After reaching a smuggler's port and managing to get information on Tennyson's whereabouts, the trio hires a ship to take them to Picta.

En route to Ran-Koshi, Horace's party befriends the local woodworkers known as Kikori, and they join his group on the trek to the fortress.

Years after the events of the previous books, Will Treaty tries to cope with the death of his wife Alyss, who died in an inn set on fire by the gang leader Jory Ruhl.

When Will's friends begin to notice the change in his personality, Gilan, the new Ranger Commandant, calls on Halt, Pauline, Cassandra, and Horace to discuss the situation.

Six months later, Maddie is awarded her bronze oakleaf, marking her as an official Ranger's apprentice, and Cassandra offers her reinstatement as a princess.

[6] The mythical world of the story is based on England, Europe, and Scandinavia because Flanagan was inspired by "English and European culture and history".

[6] Celtica's mining culture is like Wales, while Gallica takes its name and language from medieval France in its chivalric age around the year 1300.

[9] Rights to sell Ranger's Apprentice have gone to 16 countries, including North America (where Oakleaf Bearers is better known as The Battle for Skandia), the United Kingdom, France, Germany, the Netherlands, Italy, Denmark, and Sweden.

[6] One review noted that "themes of loyalty, bravery, endurance and friendship leave young readers in safe hands as they learn to navigate dangerous waters on their own.

[12] A letter was sent to the kids from the voice of Will, actually written by John Flanagan, which stated they would learn "all the skills I was taught by Halt [Will's Ranger mentor] in the woods around Castle Redmont", including "archery, tracking, and the art of concealment and unseen movement.

"[12] The 75 campers were divided into 12 fiefdoms where they were taught the skills and at the end, every kid received a T-shirt, a silver oak leaf pin, and a gold-embossed Ranger's Apprentice bookmark.

"[14] A reviewer from Booklist stated "Will's vivid world will entice fantasy readers who are drawn by the lure of high adventure carried out by believable, down-to-earth heroes.

"[15] A review by School Library Journal on Oakleaf Bearers praised the book on the high tensions raised and recommended it to people who enjoy action and adventure.

[10]John Flanagan stated in a Facebook Q&A in 2014 that the funds for the film had been acquired and auditions would be held in the summer of 2015, in either Ireland or England.

[24] In January 2024, it was reported that Skydance Media had acquired rights to the film with Theodore Melfi set to write the script, direct and produce.