Where Nolan stays true to The Odyssey – and where he takes liberties

To adapt The Odyssey for film is one heck of a massive ask, given its scale and complexity as storytelling, not to mention that we’re dealing with one of the great foundational epics of Western literature. Christopher Nolan, characteristically, is undaunted. His film is nothing if not an ingenious feat of adaptation, compressing 24 books of Homeric epic (which I pored over for my Classics degree) into just shy of three hours.

To adapt The Odyssey for film is one heck of a massive ask, given its scale and complexity as storytelling, not to mention that we're dealing with one of the great foundational epics of Western literature. Christopher Nolan, characteristically, is undaunted. His film is nothing if not an ingenious feat of adaptation, compressing 24 books of Homeric epic (which I pored over for my Classics degree) into just shy of three hours.

Bear in mind that it takes Odysseus (Matt Damon), survivor of the Trojan War, around a decade to travel the 5,000 miles back to his homeland of Ithaca, buffeted and blown off-course almost every step of the way, and losing every one of his companions. Nolan has found a path through the narrative that few other filmmakers have even tried, incorporating most of the major seafaring episodes, and concentrating equally on the Ithacan palace intrigue caused by the hero's long absence. Nolan almost made Troy (2004) for Warner Bros, meaning that some of the imagery here – including the wooden horse, half-submerged in the sand – has been tumbling around in his imagination for decades.

He has sculpted, and tinkered, and allowed himself a fair bit of creative licence to bring us his Odyssey. Here – with spoilers – is what's in and what's out, what he's added, a few things he's missed, and guesswork at the logic behind the changes.

What's in 'Zeus's law' of hospitality to strangers Xenia is the ancient Greek concept of ritualised hospitality: essentially, an obligation to treat strangers at your doorstep the way you would wish to be treated yourself. Zeus, king of the gods, was the patron of looking after strangers, and any breach of xenia was often thought to be subject to divine punishment. It was not only incumbent on hosts to provide shelter, food, drinks and so on, but an important rule for guests was not to take liberties or overstay their welcome.

Nolan has underlined xenia as one of his guiding themes, because there are multiple infractions of it in The Odyssey, especially by the unruly suitors swarming around the palace in Ithaca. Penelope's many suitors and constant weaving These parts are very much true to Homer. The power vacuum left by Odysseus's disappearance for so many years has left the throne of Ithaca empty, with his young son Telemachus (Tom Holland) itching to occupy it.

His mother, Penelope (Anne Hathaway), is unwilling to accept the probable death of her husband. From far and wide, opportunistic noblemen have flocked to seek her hand and gain the royal seat, including from neighbouring islands such as Dulichium and Zakynthos. They take up residence in Ithaca's palace, carousing, emptying the store cupboards and generally making a mockery of xenia.

Homer specifies that of the 108 suitors, just 12, including the chief schemer, Antinous (Robert Pattinson), hail from Ithaca itself. To delay having to make a choice among them, Penelope spends her days in seclusion, feigning grief while weaving a burial shroud for her husband, having promised that when she completes it she will marry one of them. Every night, she unravels her progress to buy additional time, something to which Antinous is becoming wise.

The key gods There were 12 Olympian gods among the more than 3,000 that scholars have counted overall. Nolan has kept Homer's emphasis on three key deities. Athena, goddess of wisdom (played by Zendaya), the mentor and guide of Odysseus, whose aim is to see him safely home.

A late twist that Nolan has added reveals why Athena has appeared to him in the form she does: we see a flashback to a young Trojan woman, also played by Zendaya, being slaughtered in front of Odysseus's eyes. Poseidon, god of the sea. Unlike Athena, he is given no face in the film, even if he's the primary antagonist in Homer, whose fury is incurred when Odysseus blinds his son, the Cyclops.

From that episode onwards, the voyage becomes murderously rough, with the god doing everything in his power to thwart the homeward journey of Odysseus and his crew. Zeus, king of the gods. Again, faceless.

An arbiter, who not only permits Poseidon's vengeance to play out, but also listens to the sun god Helios (a Titan, not an Olympian) when his sacred cattle are slaughtered on the island of Thrinacia. By Helios's request, Zeus destroys Odysseus's ship with lightning as a punishment. His interventions eventually cease, enabling Athena to get her way, release Odysseus from the influence of the nymph Calypso (more of which later), and allow him to wash up on Ithaca's shore.

What's out The joke in the Cyclops episode Nolan has lost the chit-chat from Odysseus's encounter with the one-eyed monster, the Cyclops. When the Cyclops Polyphemus (Bill Irwin) traps the wandering Greeks in his cave, he devours two. In Homer, he also talks to them.

This enables Odysseus to use a piece of trickery on him, when the Greeks offer him wine and their leader introduces himself as "Nobody". In Nolan's version, they don't even realise Polyphemus can speak until they have already blinded him and are planning their escape. His only words (mumbled and unintelligible) turn out to be a prayer to his father, Poseidon, for vengeance.

The creature's design is so expressive that Nolan may have decided speech was redundant; or else the "Nobody" gambit was a tough one to dramatise. We see no evidence of any neighbouring Cyclopses, to whom Polyphemus cries out vainly in Homer: "Nobody has blinded me." The cannibals In Homer, these are the Laestrygonians, a tribe of giant cannibals whom the crew fatefully encounter in book 10.

They are not cannibals in Nolan, but illustrious, faceless knights in plate armour – perhaps his biggest creative licence in terms of credible costuming, since they depart from the Bronze Age look otherwise crafted by designer Ellen Mirojnick. After the terrors of the Cyclops, which bites off several soldiers' heads, perhaps there was a worry about repetitive man-eating in too many episodes, so these guys are suited up instead with huge swords. The lotus eaters and longing for home Odysseus's seven-year stay with Calypso is the final but by far the longest delay he faces on the voyage home to the island of Ithaca.

In Homer, Calypso – the daughter of a sea nymph and a Titan – is a bewitching minor goddess who holds Odysseus captive for seven years. He weeps every day on the shore, yearning to return to his homeland, wife and son, despite the paradise Calypso is offering, with promises of eternal youth. Nolan has presented this differently.

He has merged the Calypso section of the epic (book five) with the part in book nine when Odysseus and his crew meet a tribe of "Lotus Eaters", whose addiction to a delicious, intoxicating fruit causes memory loss. (It's Odysseus who must rescue his crew from its charms, dragging them back to their ships.) Because Nolan's Calypso (Charlize Theron) is now the one feeding the plant to Odysseus alone, he forgets all about his past, and must wean himself off the fruit to remember anything.

Nolan's choice to do this is a double-edged sword. To have Odysseus recover in fits and starts, prompting flashbacks to what he newly remembers, is a handy structuring device that disperses these Calypso scenes across some two hours. But the fundamental ache for "nostos" (homecoming) – the pull towards home from which our word "nostalgia" comes – is vastly diminished by wiping the hero's memory clean for so long.

What's extra The tales of the twin wives Helen and Clytemnestra Lupita Nyong'o (whom we are yet to catch a glimpse of in the film) has been double cast as the two queens. Neither character features hugely in Homer's Odyssey, even if they are crucial figures in the surrounding mythology, whose stories the film, albeit briefly, weaves in. According to the legend, Helen and Clytemnestra were half-sisters from the Spartan royal household.

It's... complicated. Queen Leda of Sparta was seduced by Zeus in the form of a swan, and birthed two eggs, each containing a pair of children: Clytemnestra and Castor, the mortal spawn of King Tyndareus; and Helen and Pollux, the demigod children of Zeus.

Clytemnestra and Helen then respectively married the Mycenaean brothers Agamemnon and Menelaus, establishing the Greek world's two main power couples. The famously beautiful Helen is snatched by the Trojan playboy prince Paris, launching the thousand (Greek) ships to descend on Troy, as the primary cause (or, according to Nolan's screenplay, pretext) for the invasion. Meanwhile, because of Agamemnon's sacrifice of his daughter Iphigenia to obtain a favourable wind for the voyage, Clytemnestra's greeting on her husband's long-awaited return is to murder him, with the help of her lover, Aegisthus.

In turn, she is murdered by her remaining children, Orestes and Electra – a sequel outside this story's scope, but one that obsessed the Greek tragedians Euripides and Aeschylus. Sinon the Greek soldier and other character tweaks A few characters have been added. The main one is Sinon, Elliot Page's character, a Greek soldier (and cousin of Odysseus) who appears not in Homer but in Virgil's Roman foundational epic, the Aeneid, instead.

Traditionally, he's the one who tricks the Trojan forces into taking the wooden horse inside their gates, claiming to be a deserter and allowing himself to be captured, then secretly unlocking the horse in the dead of night. Nolan has both added Sinon into the mix and changed his function: in the film, he is impaled on the horse and dies while the Greek troops quietly lurk inside. When Odysseus sails to the edge of the world and summons the fallen dead, Sinon then returns as an embittered shade, despairing that his life has been wasted on a treacherous, greedy ruse.

Page is the second face we see on screen. The first is the US rapper Travis Scott, who plays an unnamed poet recounting the fall of Troy – presumably based on the Ithacan bard Phemius. The swineherd Eumaeus (a wonderful John Leguizamo) has been made visually impaired, which makes sense of his failure to recognise the returning Odysseus.

Meanwhile, Pattinson's Antinous has been given a backstory about attempting to put himself forward for the mission against Troy, but being forced by Odysseus to stay behind, triggering resentment and thereby giving a shorthand explanation for his later villainy. The happy ending, and Odysseus as an out-and-out good guy Nolan gives weight to PTSD among the battle-hardened Greeks and the remorse Odysseus expresses for his role in ransacking Troy. These are anti-martial, pacifist themes, somewhat familiar from the close of Nolan's Oppenheimer and making the two films speak to each other.

While these ideas are drawn from what's in the text, Odysseus's conscience is much more openly troubled in the film, and sooner. A long scene exploring this, between him, Penelope and an eavesdropping Athena, happens pointedly before his dust-up with the suitors, which realigns his motivation as a soul-searching hero, and sets up an ending where we feel harmony and justice have been fairly restored. Not so in Homer.

The bloodlust and savagery of the massacre at the palace of Ithaca, in which Telemachus and the returning Odysseus both mutilate their victims and go absolutely bananas, can't be overestimated. Indeed, it very nearly leads to a civil war in the final book (24), because the suitors' families are so outraged by the indiscriminate slaughter. Only the intervention of Athena, commanding both sides to lay down their weapons, is able to instil peace.

(The god-heavy book 24 and also book 23, in which a sceptical Penelope finally accepts Odysseus is who he says he is, have here been amputated.) You could argue that Nolan and Homer both arrive at a similar destination by different routes – Nolan through interiority and measured heroics, Homer by demonstrating a cycle of violence spinning way out of control. It would certainly be more disturbing for a contemporary viewer to witness the extraordinary amount of blood Homer's Odysseus spills, or watch him order his disloyal maids to clean it all up, before hanging them.

Homer's Odyssey, it's fair to say, ends on quite a downer, and to approach that faithfully would have alienated us from the surviving characters. Nolan's version, more palatable if less confronting, makes a haunted Odysseus absorb his own story's moral gist before it's even quite over.