After a long wait, and one brief trip to the Victorian age last year, Sherlock finally returned for its fourth season on New Year’s Day. While audiences were shocked (and maybe appalled) by the various twists and turns the series has now taken, there was one constant among all the change. Just as has been true since 2010, this episode of Sherlock was rife with literary references to the original 56 short stories and four novels written by Sir Arthur Conan Doyle which spawned one of the original fandoms back in 1897.

From the title of the episode to the plot itself and the various machinations involving Mary Watson, there were some huge references to the original canon. Thrown in for good measure were also some quick shout-outs to more obscure Holmesian lore, and even a few things (including a non-canonical Holmes family member) hidden perhaps in plain sight. You saw “The Six Thatchers,” but you did not observe. Here are 14 easter eggs writer Mark Gatiss slipped into the debut of Sherlock’s fourth season premiere.

Major spoilers follow.

LEFT: Sherlock with a bust of Thatcher. CENTER: Toby takes his time. RIGTH: Sherlock, you know nothing.

“The Six Napoleons”

The plot of “The Six Thatchers” draws heavily from the Doyle story “The Six Napoleons.” In the original text, numerous busts of Napoleon are being mysteriously shattered around London; here, it’s busts of Thatcher. The original story concluded with Sherlock Holmes discovering that the busts weren’t being smashed for political reasons, but instead, because one of the busts contained a hidden pearl, the Pearl of the Borgia. In the new episode Sherlock expects to find the same pearl hidden in one of the busts, but finds something else instead.

Toby the Dog

Holmes and the two Watsons employ a dog named Toby to help track down some scents in the new episode. Toby is a direct reference to a dog named Toby from the novel The Sign of the Four. In that story Sherlock says, “I would rather have Toby’s help than that of the whole detective force of London.” In the episode, Toby is also a bit indecisive, which is a reference to the book version of the dog, too. As Watson, Doyle writes that Toby was, “the very picture of canine indecision.” Toby the dog also featured prominently in Disney’s The Great Mouse Detective.

Sherlock Holmes Knows Nothing About Politics

In “The Six Thatchers,” Sherlock frequently acts as if he is completely unfamiliar with former Prime Minister Margaret Thatcher. This ignorance of politics is a reference to Doyle’s canonical Sherlock Holmes having huge gaps of knowledge in several areas which were of no interest to him. In the very first Sherlock Holmes novel ever, A Study in Scarlet, Watson outlines a list of Holmes’s “limits.” One of the items reads like this: “Knowledge of Politics: Feeble.” Sherlock previously referenced the detective’s other knowledge gaps in the first season episode “The Great Game,” in which Sherlock’s ignorance of astronomy was key.

LEFT: Cecil. CENTER: Reigate reference. RIGHT: Mycroft phones...his other brother? 

Cecil Hotel

When Mary is undercover, she is eventually discovered by Sherlock in a building called “Hotel Cecil.” This is probably a reference to the canonical Mary Morstan working as a governess for Mrs. Cecil Forrester prior to marrying John Watson at the end of the Doyle novel, The Sign of the Four.

“Reigate Square”

Toward the end of the episode, Mycroft looks casually at a take-out menu for a restaurant called “Reigate Square.” This references the Doyle story “The Reigate Squires,” which in American editions was sometimes published as “The Reigate Puzzle.” The story is notable insofar as it depicts Sherlock Holmes somewhat off his game, which is certainly true by the end of this episode, too.

Sherrinford Holmes

In the same scene, Mycroft asks to be put through “to Sherrinford.” This has led some fans to believe the third Holmes brother, Sherrinford Holmes will be revealed in this season. Mycroft vaguely referenced a third Holmes sibling in “His Law Vow,” when he made mention of “the other one.” Since Mycroft doesn’t actually say “let me speak with Sherrinford,” its unclear if this version of Sherrinford is a person. It’s also important to note that “Sherrinford Holmes” never actually appeared in any of the official Doyle stories, but that “Sherrinford” was Doyle’s first idea for a name for the character who would become Sherlock Holmes.

LEFT: A thumb on ice. CENTER: 'The Canary Trainer.' RIGHT: Sherlock delivers one of his famous phrases...to a baby.

“The Engineer’s Thumb”

In one of the montages in which Sherlock is solving numerous cases he mentions “the wrong thumb.” This is a reference to the classic story, “The Engineer’s Thumb,” one of the few Doyle stories in which Watson takes the spotlight.

“Canary Trainer”

Also mentioned during the numerous-cases montage, Sherlock says something about a “Canary trainer.” This is a double reference to an unchronicled case only alluded to in the Doyle story “The Adventure of Black Peter,” but it’s also the title of Nicholas Meyer-penned Sherlock Holmes pastiche of the same name. Meyer’s other famous Holmes pastiche, The Seven Per-Cent Solution, was referenced in “The Abominable Bride” last year.

“You See But You Do Not Observe!”

Sherlock delivers this famous line to John and Mary’s infant daughter Rosamond. In the Doyle stories, Sherlock utters this axiom in “A Scandal in Bohemia,” where he is admonishing Watson’s inability to know how many steps there are up to their flat on Baker Street.

LEFT: Vampire text. CENTER: Culverton Smith RIGHT: Lestrade

“The Sussex Vampire”

Watson’s mistress mentions being a vampire in a text message, which might be a sly reference to the story “The Sussex Vampire.”

Visage of the Bad Guy in the Next Episode

In the forthcoming Sherlock episode — “The Lying Detective” — Toby Jones is playing Culverton Smith, a villain from the Holmes story “The Dying Detective.” He’s viewed very briefly here on the side of a bus stop, seemingly the star of an in-universe TV show.

Giving Lestrade Credit

“The Six Napoleons” is one of the rare Doyle stories in which Holmes seems very fond of working with Scotland Yard, making Lestrade’s team-up with Sherlock in “The Six Thatchers” totally appropriate. As with many stories, Holmes suggests the official credit be given to the police force, which is subtly mocked in this episode since Lestrade mentions that getting credit doesn’t matter at the point at which John will publish the story on his blog.

Mary's super-secret commando team, also a 'Sign of the Four' reference

Agra Treasure and The Sign of the Four

The super-secret memory stick — which first appeared in “His Last Vow” — returns in “The Six Thatchers.” This references the novel The Sign of the Four in two ways. First: though the initials “A.G.R.A” are now revealed to stand in for Mary’s fellow assassins, the word “Agra” references the “Agra Treasure” from The Sign of the Four. That novel’s premise also deals with the idea that four people had equal knowledge of a secret, which is how the memory sticks play out here too: mutual blackmail.

Death of Dr. Watson’s Wife

Showdown at the aquarium. Not a reference to the play and film 'Closer.'

The surprise ending of “The Six Thatchers” found Mary Watson talking a bullet for Sherlock Holmes. While this is a very dramatic twist, the death of Dr. Watson’s wife is in itself a reference to the Conan Doyle stories. In between Sherlock Holmes’s fake death in “The Final Problem” and his return in “The Empty House,” John Watson’s wife — Mary Morstan from The Sign of the Four — did indeed die. Writing as Watson, Doyle never made it clear how and why Mary died, but this “twist” was bizarrely faithful to the facts of the show, too. Just as the literary Watson didn’t write about his wife’s death, the audience can’t imagine TV Watson putting this particular heartbreaking story on his blog.

Episode 2 of Sherlock, “The Lying Detective,” airs on Sunday, January 9 on BBC and Masterpiece Mystery on PBS.