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Short version
Upon his canonical death, Severus Snape meets Albus Dumbledore in the afterlife and is given the opportunity to return to his sixth year of Hogwarts and try again to avoid his many regrets. He accepts in hopes of keeping Lily alive this time.
Throughout sixth year, Severus works with Dumbledore to begin tracking down the Horcruxes (quickly retrieving the Gaunt Ring and Hufflepuff's Cup), and wins the respect of Slytherin House, using his influence to lead them away from the Death Eaters. Lily, and the Marauders, are initially suspicious of him, but after being convinced of his good intentions, Lily forgives him, and they later begin dating, while the Marauders remain at arm's length but end open hostilities.
In seventh year, Lily and Severus are married, earlier than originally planned due to her father's untimely death. Severus and Dumbledore also successfully lure Voldemort into sending the Diary to school, where it and the basilisk are destroyed.
After graduation, Severus stays on as the Defence teacher and Head of Slytherin. Lily is kidnapped by Death Eaters as a trap for him; he rescues her, but loses his right hand, and Voldemort learns that he has knowledge of the future. Severus tells Lily about how he came back in time, and she joins in helping Dumbledore to turn the Sword of Gryffindor into a trap. Dumbledore also locates and cleanses the Diadem of Ravenclaw.
Instead of Regulus Black, Severus and Dumbledore go to retrieve Slytherin's Locket, but Voldemort ambushes them with a group of Death Eaters, retrieves the Locket himself, and succeeds in killing Dumbledore (weakened by the protective potion). However, his attempt to then turn the Sword of Gryffindor into a horcrux fails due to Lily's enchantment, leaving him without enough remaining soul to make another, and Voldemort retreats. Severus then deliberately leaks a false prophecy about Voldemort's downfall (as Voldemort believes him to be a seer), causing Voldemort to march on Hogwarts. Despite heavy casualties, the defenders are able to obtain reinforcements from the Aurors, and Lily and James, working together, finish Voldemort off with the Sword of Gryffindor. Severus is hit by a Killing Curse and visits the afterlife again, where he makes peace with an adult Harry, but is able to return to life due to his wedding ring, which has been set with chips from an alchemical Death Focus Stone and a Philosopher's Stone, and thus (inadvertently) prevents his soul from being ejected from his body.
The epilogue shows Severus and Lily raising their baby boy together, whom they name Albus Harry Snape, and looking forward to the future.
Longer version
Severus Snape arrives in the afterlife upon his canonical death, and meets Albus Dumbledore, who informs him that since his past has so many regrets, he has the opportunity to go back in time and try to remedy them — but at a point when his breakup with Lily has already occurred. Dumbledore gives him some of his own memories, which Snape agrees to take to the Dumbledore of the past.
Severus then wakes up in his sixteen year old body, at the start of his sixth year at Hogwarts. He approaches Dumbledore, who views the memories he has brought, which include the imprisonment of Albus' father, Albus' infatuation with Grindelwald, and the death of Ariana, as well as partial knowledge about the Horcruxes, and he provisionally accepts Severus' claims about coming from the future. He requests Severus' own memories of each war, so he can begin planning, and is quickly able to obtain the Gaunt ring, without harm to himself this time, although he delays destroying it, hoping to find a method that will not ruin the Resurrection Stone. He also arranges for Severus to begin learning alchemy, both to keep his mind active despite the ease of his class work, and to provide a cover story for them to meet regularly.
Lily herself is cold toward Severus, though still hurting about the breakup of their friendship. She quickly notices that he is acting differently, still harsh but more self-controlled and reserved. He then proceeds to disrupt the politics within Slytherin House, demonstrating his magical superiority over the aspiring Death Eater recruits who currently hold influence, and positioning himself as the de facto leader, with many students acknowledging his skill and following him — much to the dismay of Lily, who believes him to be preparing to serve Voldemort, and the Marauders, who repeatedly confront and challenge him, though he continues to hold back and try to ignore them.
When Severus publicly announces a policy that Slytherins must always protect and help others of their House, he wins even more support from the younger students, but the prospect of a united Slytherin is so alarming to the other Houses that the Marauders resolve to publicly humiliate him and thus undermine his efforts. However, by the time their planned ambush takes place, Severus' policies have taken root, and a younger student comes to his aid, allowing him to escape their trap and turn the tables on them. He chooses to let them go, declaring in front of many witnesses that he is not leading Slytherin toward joining the Death Eaters, which shocks everyone and internally divides the House; some, especially the younger years, continue to follow Severus and his promises of safety and unity, while those who are more devoted to Voldemort separate themselves and oppose him, and yet others remain neutral.
Lily then seeks Severus out to question him, and is delighted when he confirms that he meant what he said, that he has turned his back on Voldemort. She forgives his earlier actions and they reconcile as friends, to his astonishment. After several weeks, with the Christmas holiday approaching, James eventually comes to speak with him as well, seeking to confirm his story and establish a cease-fire; Severus is angered by his presence and uninterested in his apologies, but very grudgingly shakes his hand when Lily arrives, so as not to disappoint her.
Before the break, however, Dumbledore becomes concerned about Severus' lack of remorse about having killed his parents in the first timeline, and his expressed lack of concern if they died again. Severus avoids him during the holiday, attempting to make progress in alchemy despite not having the necessary ambidexterity; when they meet again, Dumbledore expresses a worry that he is only loyal to the cause because of Lily's death, and might turn away in this timeline if he could take her with him. Severus storms off, and Dumbledore leaves out the Pensieve containing his memories, to remind Severus of what matters when he returns — but before he does, Lily comes across the Pensieve, and learns about his first life.
Severus is shocked and alarmed by what this knowledge means for Lily, but tells her honestly about his first life and his mistakes; she is saddened by a lot of it, and especially disappointed that he didn't love Harry, but is also moved by his unyielding devotion to her, and asks him to teach her the Patronus charm that represents their connection. Over the following days, while learning the charm, she also reflects at length on her feelings about him and about James Potter, and determines that she isn't willing to give Severus up to be with James, publicly telling James that she chooses Severus. Unfortunately, her knowledge about the future would make her a target for Voldemort, and Severus realises that her memories need to be removed; she's upset by the prospect, especially the way it will set their relationship back, but agrees on the condition that he'll restore them someday, and that he promises to still pursue her and not give up on her. Seeing what Severus goes through in doing this, Dumbledore acknowledges his devotion and no longer worries about him turning away.
Without her memories and the knowledge of his feelings for her, Lily's relationship with Severus cools back into simple friendship, and their Patronus lessons are put on hold, but James accepts her stated choice (despite her no longer remembering it) and backs off. In keeping with his promise, however, when Severus sends her a birthday gift (an unbreakable bottle containing her memories, shaped into a doe), he signs it with "Love", which sends her feelings into turmoil. When their Patronus lessons resume, she makes enough progress to recognise that both their Patronuses take the same form, and questions him about it; he admits that he loves her, which takes her aback, and she leaves without being able to immediately answer, to his great chagrin and disappointment.
A short time later, some of the seventh-year Slytherins ambush Lily in a corridor, to punish Severus for leading Slytherin House away from blood purity ideals and Voldemort, and the Marauders help to rescue her. To repay them, Severus brews the Wolfsbane Potion for Remus, which he had previously refused to do because of their feud; Sirius still mistrusts him, but James is impressed. On the night of the full moon, Lily tells Severus to meet her by the lake at night, so they can watch Remus' transformation. Severus is shocked by the risk and runs after her to stop her, but Remus remains safe, and Lily tells Severus how pleased she is with his change and how noble he's become, then casts a corporeal Patronus, her doe, to show that her heart matches his. They share their first kiss under the moonlight — to the dismay of many of her friends, who dislike and mistrust him. Her father and sister, over the Easter break, also express disapproval.
Severus is reluctant to keep brewing Wolfsbane on an ongoing basis, as it is a major investment of time and effort, but agrees to do so because Lily asks him to. This alarms Remus when he learns about it, as he worries that Severus is using it as leverage to force Lily to stay around him; he expresses his concerns to Severus, who is enraged by them, but also worries that it might be true in practice, and starts teaching her to brew it herself so that she is not dependent on him. She appreciates his thoughtfulness, but later learns that Remus spoke to him and is upset by the meddling. In light of Severus' recent behaviour, James attempts to apologise to Severus for their past treatment of him, but Severus angrily turns him away; Lily overhears, and recognises Severus' hurt, but encourages him to forgive for the sake of having peace within himself. Severus is reluctant, but unwilling to punish her for his own guilt or James' actions, and agrees to try.
Dumbledore, meanwhile, has retrieved the Cup horcrux and destroyed it with Fiendfyre. With Severus' growing alchemy skill, including the invention of a viable one-handed transmutation technique, Dumbledore asks him to research transmuting Basilisk venom, in hopes of using it to cleanse the Horcruxes less destructively. Severus also succeeds in creating a Focus Stone for transmuting copper, by crystallising a memory of facing death with fortitude. Nicolas Flamel invites him to an alchemical conference to present his one-handed transmutation technique, but Severus begs off due to lack of funds and dislike for public speaking.
Lily's friend Marlene continues to disapprove of Severus, leading to a fight between them where Lily reveals Marlene's crush on James Potter in front of witnesses. Marlene is upset and shuns Lily, who seeks out comfort (and some advice) from Severus. In the process, a misunderstanding causes him to push their encounter toward more physicality than she's ready for, but he's mortified when he realises his mistake; Lily talks to a friend whom she can trust to be discreet, and visits him again later to reassure him there's no harm done. She's also able to get James and Marlene to talk to each other about Marlene's feelings, though James is unsure about his own.
Severus becomes aware that Regulus Black has taken the Dark Mark, but pretends to be unaware of its significance, though he reports it to Dumbledore. Dumbledore chooses not to take action, since knowledge of the Dark Mark is not yet common, and gives Severus a different task over the holidays: convincing his grandfather, Amadeus Prince, to reinstate Severus' mother and thus keep the Prince inheritance away from Voldemort. Severus does choose to speak to Regulus, however, telling him that they need not be enemies while they are at school, and that he is willing to help if Regulus finds himself over his head; Regulus appears uninterested for now.
Lily accompanies Severus to obtain new dress robes for approaching his grandfather (though he claims that they're for the conference that he won't in fact attend), and sees the scars on his back from his father's abuse. The realisation of what he went through, and nearly went through alone, greatly upsets her, but she finds some comfort in arranging for his new robes to include the pattern of a doe on each sleeve, picked out in silver thread.
Sirius, in his turn, comes to see Severus, not apologetic like James, but asking what it would take to bury the hatchet for James' sake. Each accuses the other of being responsible for their past feud, and Severus sends him away.
Around the same time, Marlene and James start their relationship off with a bang, which helps Marlene reconcile with Lily, although she remains dismissive of Severus, and her relationship with James only intensifies her claims that Lily could do better. Although Lily remains attached to Severus, she does find herself envying the easy and open affection of Marlene's relationship. They're able to talk about it at a party thrown in Gryffindor Tower, with all the other Houses invited as a gesture of unity; Severus is surprised by her desire for him to be more affectionate, and it doesn't come naturally to him, but he's not displeased.
Over summer, Severus visits the Evans home to prepare the Wolfsbane Potion there. However, Mr Evans disapproves of his relationship with Lily, because of how he hurt her and the risk that an abused child may abuse others in turn. Lily is upset by this attitude, but troubled by the possibility that he's right; she and Severus are able to spend more time together, though, and she is inadvertently present when Amadeus Prince is taking Eileen back. Severus respectfully but firmly chooses her over his grandfather's offer to take him too, which angers Amadeus, but Severus is resolute, reducing Lily to tears afterward. Severus' father then walks in on them, belittles Severus, and reacts with anger to the news that Eileen has left him, striking both of them before Severus furiously disables him with a cutting curse. Despite Severus' panic about Lily having been in danger, she's able to calm him down and he does what he can to treat his father's wounds, after which his father turns him out of the house.
Lily visits Severus in his bedroom that evening to comfort him, expecting their relationship to become intimate, but he's unwilling to go there under her father's roof, and she simply falls asleep next to him (after which he moves her back to her own bed). She's a little disappointed, but her father finds out and approves of Severus' decision (which she finds further disappointing). Dumbledore grants Severus the use of his own family home, which has remained empty for 50 years, and appoints Severus and Lily as Head Boy and Girl.
As the new school year begins, Lily continues to flirt with Severus, which he likes but struggles to relax and enjoy it, including her jokingly asking him when their wedding is. When she later takes him to see the Room of Requirement, it forms itself into a family home; Severus admits to her that he would be quite unable to afford a home like it, or indeed any home, but she assures him that that doesn't matter to her. He proposes with a ring formed from the silver produced in his alchemy studies, and she accepts.
Several of Lily's friends are quite alarmed by this development, even Mary, who supported her earlier efforts, and especially Marlene, who insists that Lily is too good for Severus. However, Lily stands her ground, and Mary is mollified when Lily is able to persuade her that she's thought it through and is sure of what she's doing. Lily's father also accepts that Severus has changed, based on his observations over the summer, and gives his blessing. Lily persuades Severus to make a matching ring for himself, so that both can be enchanted; the resulting magic allows them to sense and find each other at a distance.
James visits Severus, seeking a truce again. This time, though Severus is still bitter and angry, he's more strongly influenced by the desire not to see Lily upset, and the knowledge that she's chosen to be with him; James is able to persuade him to lower hostilities so as not to upset her. Severus and Lily later meet with Petunia and her fiance Vernon, which ends badly when Lily stops putting up with Petunia's barbs, but Severus makes a tolerable impression.
Dumbledore lets Severus know that he has been unable to persuade the Grey Lady to tell him anything about Ravenclaw's Diadem, and he is reluctant to compel her using his authority as Headmaster unless they run short of time. Severus approaches the Bloody Baron, with Lily following him, which results in Lily learning that the Grey Lady is Helena Ravenclaw, but the Baron retreats into his melancholy and gives no further help. Lily later meets the Grey Lady, weeping with her about her sad past and encouraging her to forgive herself.
At Halloween, Severus leaves the feast early, tormented by the memory of Lily's death twenty (subjective) years earlier. Lily follows him using her ring, to find out what's troubling him; unable to tell her the full truth, he admits to having nightmares about her dying and it being his fault.
A few weeks later, they attend Petunia's wedding together, though Petunia is unhappy with them and deliberately shuns Lily. The event also reminds Severus of his own poverty and inability to provide something similar. He stays at the Evans home overnight, where Lily overhears him having another nightmare; she joins him in bed to cuddle and comfort him, much to his shock in the morning, although nothing further happens.
By Christmas, Severus' dour but efficient efforts as Head Boy, his influence on Slytherin, his brewing of Wolfsbane, and the fact that his relationship with Lily is making them both happy, have caused several of Lily's friends to warm to him; even Marlene decides that he's "not a bad guy". However, just before they return home for the Christmas holiday, Lily receives word that her father has passed away of a heart attack.
Severus accompanies Lily to the funeral. Both are deeply saddened by the loss, but Lily is able to achieve a measure of reconciliation with Petunia. Severus is unable to understand how they can laugh at reminiscences of their father, not having any experience of healthy grieving himself; when she explains what she's going through (not realising that he has grieved before), he's astounded that she can understand better than him despite being so much younger.
Lily then surprises him by pointing out that the reasons to wait until after graduation for their wedding have changed. After selling her family home to pay off the mortgage, there won't be much left, and she'll have little except the clothes on her back, plus it's no longer possible for her father to give her away, so her ideal wedding would never happen in any case. Severus is hesitant, but agrees to her wish to be married before the new year, so they can at least start out with each other even if they have nothing else. Petunia refuses to attend, and it's merely a short exchange of vows with no fanfare, but Lily is happy.
Upon returning to school, Lily's friends are shocked by their sudden action, but begin to come around. Marlene, however, lets Lily know that the Order of the Phoenix is recruiting, and that she plans to join after graduation. Lily attempts to raise the matter with Severus, but he harshly forbids her to risk her life in such a way, angering and upsetting her. They are able to talk about it and reach a tentative agreement that she won't risk her life, but the matter remains unsettled.
Dumbledore, meanwhile, has taken steps to provoke Tom, using future knowledge to intercept and thwart an attack on the Proud family. He reveals to Severus that, besides saving their lives, his intent was to provoke Tom into sending the Diary to the school to retaliate against him. Severus notices that Rosier is behaving unusually, and Avery has met Riddle face to face earlier than he did in the original timeline. Severus continues to work on transmuting basilisk venom to destroy Horcruxes, but in the course of his experimentation, he produces something else: a Focus Stone produced from a mixture of common snake venoms and his memories of the afterlife, which gives him a glimpse of his own soul (including the Dark Mark) when he holds it, but does not have the same effect for anyone else.
Lily then mentions to Severus that some of Hagrid's roosters are being killed, and he immediately alerts Dumbledore. He is then forced to reveal to Lily their significance, so that she'll agree to him being stiflingly protective, and she alerts her friends. Despite Severus' desire to immediately snatch the Diary and destroy it, Dumbledore persuades him to wait for the Chamber to actually be opened, so that they can kill the basilisk as well. Shortly afterward, the roosters are finished off; knowing that the basilisk will soon be unleashed, Lily evacuates the students to the Room of Requirement, while Severus goes to fight. He finds that Rosier has been ordered to hinder him but is scared to actually try it, and Severus persuades him to stand aside, while Avery is possessed by the Diary and is controlling the snake. To Severus' surprise, his Death Focus Stone is able to destroy the Horcrux attached to the Diary, and he then finishes the basilisk off with Dumbledore's help.
Lily and Severus then graduate, with James anonymously supplying Severus with new dress robes in recognition of his heroic efforts; Severus deduces their origin and is angry, but chooses not to make a scene for Lily's sake. He is offered an apprenticeship with Damocles Belby as a result of his contributions to the Wolfsbane recipe, but accedes to Dumbledore's wish to make him the head of Slytherin House instead, teaching Defence Against the Dark Arts, and they move into the Dumbledore family home together, partly for their safety in case Voldemort retaliates for killing the basilisk (as the house is heavily warded). Lily also finds employment as an enchanter of magical toys and trinkets.
Lily then attempts to join the Order of the Phoenix, feeling a need to contribute and protect people from the Death Eaters, despite having promised Severus that she would stay out of the war. Her application is vetoed by Dumbledore, however, at Severus' insistence, causing an argument between Lily and Severus. Severus himself does join, helping to train Order members in duelling, although he doesn't go onto the front lines. Alastor Moody becomes suspicious of his skills, recognising his combat style as similar to the Death Eaters, and questions him about it, leading to Severus admitting in front of Lily that the Death Eaters trained him. She's concerned to hear how close to them he became, but accepts his assurances that he has turned his back on them.
Rita Skeeter, however, uses her Animagus form to infiltrate the Snapes' home and find out about his past, publishing an article about his prior association with the Death Eaters. Severus traps her, and then tricks her into confessing her actions while Dumbledore is listening in; Dumbledore makes a deal with her to use her skills to serve the Order.
After the school year starts, with Severus teaching and Lily working only part time, she visits her old home to remove some traces of magic that were accidentally left behind, and in the process, meets Severus' father, who was attempting to track Severus down. Tobias is unimpressed by her, but she does let him know that she and Severus are married now, and he expresses a slight desire to keep in contact. Severus later has the opportunity to speak with Barty Crouch Jr about the latter's own difficulties with his father, learning that Crouch Sr is not directly abusive like Tobias, but has extremely high and strict expectations that are driving a wedge between them.
To further her career in enchantment, Lily applies for a Charms Masterclass certification, but her application is intercepted by Augustus Rookwood of the Department of Mysteries, who interviews her to become an Unspeakable, and upon her declining, removes her memory of the interview, leaving her only with the idea that her application was declined and rescheduled. At the second interview, she is abducted from the Ministry and a Portkey left behind. Severus follows, knowing it's a trap, but seeing no alternative; it takes him to a forest where Voldemort and many Death Eaters are waiting.
In hopes of creating an opening to evacuate Lily with the Portkey, Severus pretends willingness to join Voldemort and serve as a double agent, dismissing his marriage to Lily as a passing fancy. However, knowing how devoted he is to her, Lily sees through his ruse immediately — and Voldemort reads it in her mind, allowing him to quash Severus' plans and destroy the Portkey. He then invades Severus' mind; Severus is mostly able to hold him back, but Voldemort catches a glimpse of a memory from Severus' first life, leading him to believe that Severus is a Seer. He then uses the Imperius curse to compel Severus to kill Lily, but Severus resists long enough to cut off his own right hand to stop himself. The Order of the Phoenix then arrives, disrupting the gathering and giving Lily an opening to break free and take Severus with her; for lack of other places within reach, she goes to Spinner's End, where Tobias rouses himself enough to provide first aid and save an unconscious Severus' life.
Severus and Lily move into Hogwarts full time for their safety, with Lily assisting Professor Slughorn, and with Voldemort already after them both, Severus returns Lily's memories of the week where she learned about his first life. However, the loss of his wand hand weighs heavily on him, as he is unable to cast left-handed and thus cannot use any magic at all, not even brewing potions, making him feel like a burden on Lily, though she tries to reassure him of how much she loves him.
Severus begins practising casting with his left hand, but finds that his magic is so thoroughly accustomed to using his right hand that it does not respond. However, he does learn that the Death Focus Stone still shows his soul including the hand. He also encounters the Bloody Baron during one of his attempts, and when the ghost perceives the Stone's effect, he hands it over - discovering that the Stone can give the ghost temporary corporeality. Dumbledore is intrigued by the discovery, and tells Severus about the other bridge he has found between the living and the dead, the Resurrection Stone. Severus is furious at the discovery that in his first life, Dumbledore kept this knowledge from him, denying him the opportunity to apologise to Lily, but he is taken off-guard when Dumbledore admits to having been unfair. Dumbledore also lets him know that he hopes to have the ghost of Helena Ravenclaw use the two stones to summon her mother so that they can inquire about the Diadem.
Severus continues teaching, with James assisting in the practical classes, and slowly begins to find success in casting left-handed. He is also able to help Barty Crouch Jr to find some respite from his father, involving him in class preparations over the Christmas break, which wins him Barty's gratitude.
Helena's ghost then approaches Lily for emotional support, as she uses the Resurrection Stone. She is able to apologise to her mother, and finding peace, disappears. During the conversation, Dumbledore is also able to question Rowena about the Diadem, and from her advice, deduces its location within the castle; he and Severus retrieve it from the Room of Requirement and cleanse it with a Death Focus Stone. Dumbledore then begins experimenting with creating a silver hand for Severus, like the one granted to Peter Pettigrew in the original timeline, although he does not have immediate success.
Lucius Malfoy visits Hogwarts unexpectedly, and is found near the Room of Requirement, leading Severus and Dumbledore to suspect that Voldemort fears for the safety of his Horcruxes. Dumbledore is concerned about the prospect of Riddle making an additional Horcrux, although he believes that more than one would place too much strain on his soul and thus not be possible. Lily questions Severus afterward about his former friendship with the Malfoys, and helps Severus to recognise that he was more willing to overlook Draco's parentage than Harry's, of which he is ashamed.
During the holiday, Severus is able to learn more about the strained relationship between Crouch Sr and Jr, with the father setting impossible standards and dismissing all the boy's achievements as merely to be expected, which is pushing him toward rebellion. Although Severus' own relationship with his father is slightly improved by Tobias saving his life, he is able to have Barty open up to him, and recommends caution about burning bridges. He also has a cordial conversation with James, who persuades him to come to his wedding to Marlene.
Dumbledore then succeeds in crafting a silver hand for Severus, not using the same method as Voldemort (which would involve carving it from Severus' soul), but instead studying and mimicking the anatomy of a hand in detail. The prosthetic needs to be regularly renewed, but in conjunction with Severus' ongoing left-handed practice, it restores him to a reasonable level of spellcasting ability. Madam Pomfrey and Lily both begin to learn the necessary skills to renew it.
At the end of the holiday, Regulus Black approaches Severus, telling him about how Voldemort used Kreacher to test the protections around the Locket, and, outraged over this mistreatment of his beloved elf, seeking Severus' help. Severus and Dumbledore persuade him not to attempt revenge himself, and make their own plans to assault the cave and seize the Locket.
Dumbledore then asks Lily to learn Occlumency, in order to protect the knowledge of an enchantment that he wants to make; to Severus he explains that he wants to turn the Horcrux-killing properties of the Death Focus Stone into an enchantment, to deal with the possibility of Voldemort making an extra horcrux. Severus is displeased at the prospect of involving Lily in the war effort, but acknowledges that she may be able to help end the war sooner, and agrees to teach her Occlumency. Her progress is slow, because her mind is naturally more open than his, but she gets there with Professor McGonagall's help.
Dumbledore then seeks the Sword of Gryffindor, the last founders' relic, which he knows Voldemort will want to use for his horcrux if he can. By his arrangement, Severus helps to save James from a dragon, and James draws the sword from the Sorting Hat in the process, allowing him to decapitate the dragon. Unfortunately, both James and Severus are then arrested for poaching, and put through a rigged trial; Dumbledore intervenes and rescues them, producing the sword as evidence of their honourable and courageous intentions (and making it public knowledge in the process).
Lily continues to experiment on a series of stones, attempting to imbue them with the properties of a Death Focus Stone, until she suffers a mishap, with the Carrow siblings attacking her in Dumbledore's office and causing an unexpected interaction with one of the prototypes. It attacks Lily's soul, draining her of strength until she is incapacitated. Severus confronts the Carrows and persuades them to hand over the stone responsible, in exchange for a promise to give them the Sword of Gryffindor that they have been tasked to retrieve. Destroying the stone breaks the curse, but does not repair the damage already done; however, in a stroke of inspiration, Severus is able to use a memory of his relationship with Lily to create a Philosopher's Stone, producing enough Elixir of Life to heal her.
In the aftermath, Severus refuses to have Lily embark on the same course of experimentation again, and Dumbledore agrees, but Lily strikes on another approach, and successfully imbues the Sword of Gryffindor with the essence of the Patronus Charm. The Bloody Baron agrees to test it, and it brings him peace, causing him to disappear from the world, which Severus and Albus consider to be a successful test; they hand over the sword to the Carrows, as agreed.
Severus and Albus then go to the seaside cave containing the Locket, intending to destroy it. However, when Dumbledore has drunk most of the protective potion, Voldemort and a number of Death Eaters appear. Severus is unable to reach and destroy the Locket in time, and one of the Death Eaters succeeds in killing Albus in his weakened state, though he is then killed in turn by a furious Voldemort, who wanted to end Dumbledore himself. Voldemort then attempts to use the death to turn the Sword of Gryffindor into another Horcrux, but its enchantment causes the soul piece to simply dissipate; he angrily settles for retrieving and wearing the Locket. However, Severus retrieves the Cloak of Invisibility and the Elder Wand from Dumbledore's body, and sets the Inferi of the lake on the Death Eaters, causing Voldemort to take the survivors and retreat. The wand then recognises Severus as its master for the way that he and Dumbledore outsmarted Tom.
Upon his return, Severus learns that Lily is pregnant with a son. With Dumbledore gone, he knows that the war cannot be allowed to drag on much longer, or Voldemort will be unopposed, so he informs the Order of the Phoenix of his first life and foreknowledge. They set a trap for Voldemort, dropping information that suggests there is a prophecy that Voldemort will lose if Severus still lives in a week's time, and thus forcing him to assault Hogwarts, where the Auror force will be waiting.
Unfortunately, Voldemort attacks more swiftly than expected, and is able to create a diversion that keeps most of the Aurors from arriving at the battle, leaving only the staff and students to defend the castle. There is a tremendous battle, with many casualties, but the tide is against the defenders, until Lily uses Dumbledore's deluminator, left to her in his will; it contacts many of Dumbledore's supporters and conveys his permission to apparate into Hogwarts, allowing them to outnumber the Death Eaters and turn the tables.
Severus himself attacks Voldemort and succeeds in destroying the Locket with a Death Focus Stone, rendering Voldemort mortal, though he cannot prevent Voldemort from striking him with a Killing Curse in return. Voldemort then ambushes Lily and invades her mind, learning about Severus' first life, but is distracted by James, and Lily is able to use her wordless wandless command of magic to stab him through the chest with the Sword of Gryffindor. The surviving Death Eaters panic and flee.
Severus then finds himself in a similar limbo to the one he experienced after his first death, where Dumbledore reassures him that Riddle is dead and that Lily survived. He also encounters a forty-year-old Harry, who is currently displaced from history (as a result of the events of the Cursed Child), and they find a measure of reconciliation. Having found peace, Severus expects to pass on, but Dumbledore then informs him that as a result of his wedding ring having been set with chips of a Death Focus Stone and his Philosopher's Stone, it was able to resist the Killing Curse, and so he has the option to return to life if he so chooses; he once again accepts, wanting to be a father.
The epilogue is set one year later, and recounts how most (but not all) of the remaining Death Eaters have been arrested and convicted, including Lucius Malfoy. Severus is still teaching Defence, with the jinx on the position having broken with Voldemort's death, and Lily is preparing to take over teaching Potions and let Slughorn retire, but is focusing first on their infant son, whom they name Albus Harry Snape.
