Timeline:
Begins Summer after Fourth Year.

Alterations to Canon:
None.

Pairings:
Harry/Tonks/Fleur
Harry/Harem

Story:

Tonks has just been recruited to the Order of the Phoenix and has been assigned a rotation on guard duty for Privet Drive. One day upon arriving for her shift, relieving Dung, she notices that the house seems almost abandoned. Interrogating Dung he coughs up that the previous day the Dursleys left in their car and never came back.

Concerned that they might be guarding an empty house if Dung has been his usual incompetent and inattentive self she lets herself into the Dursley's house and gets a shock when Harry gets the drop on her from under his own invisibility cloak. After convincing him that she's there to protect him, mainly by explaining her relation to Sirius, she notices that Harry doesn't look particularly well.

The Dursleys, Harry explains, have left to go on a vacation with Vernon's sister, Marge, and won't be back for at least another week and a bit. He's been fine by himself but admits he's been experiencing a fever and not sleeping well. Tonks explains that as an Auror she knows some field medicine but hasn't got any talent for it but volunteers to check him out... and discovers that standing within a few feet of him that she really does feel like checking him out.

Harry himself is no less distracted and apologises when Tonks catches him staring at her chest. He didn't tell the entire truth when he said he hadn't been sleeping well, he'd been having increasingly erotic dreams about girls he knew, Hermione, Katie, Fleur. Masturbating half a dozen times a day barely seemed to be keeping his libido in check as he continuously got hard over and over again. A woman as sexy as Tonks is driving him up the wall.

Tonks, embarrassed by her rising desire and how much she got off on Harry ogling her, mumbles that apart from his magic surging in bursts there's nothing she can find, he doesn't have an infection or injury she can detect. Harry, wanting to put some distance between them steps away and offers to make her something to eat, opening the fridge to reveal it to be almost empty. Tonks says he better buy some groceries, Harry explains that he's got no muggle currency and the Dursleys didn't leave any, Tonks volunteers to swap some pounds she has for the galleons Harry has in his trunk.

Following Harry up to his room to make the exchange however results in close quarters again and Tonks is struck with an even greater burst of arousal, Harry likewise finding himself entranced by Tonks' beauty and minutes later they are mauling each other relentlessly.

After hours of shagging each other senseless their minds finally start to clear and it's Harry who first asks what the heck just happened between them. Tonks hums almost disinterestedly as she squirms against his side in bed. Not sure, but it was a lot of fun, her toes are still tingling. They only just met, today, and they just leapt into bed, that's not normal, not... not right, is it? Tonks, more serious now admits that it isn't and says she's not sure how she feels about it but she doesn't feel like she regrets it.

Dressing and preparing to leave Tonks explains she'll be back on duty in the afternoon in two days time... a lingering suggestive silence, if Harry wants someone to watch his back when he goes grocery shopping. Before she leaves his bedroom she stops, turns around looking at him for a long moment and then crosses the room to snog him hard. See you later, Harry.

That night Harry sleeps peacefully and though his dreams are filled with Tonks he doesn't wake up achingly hard and feverish. The next day however he becomes increasingly aroused and frustrated as time goes on and has to wank a couple of times in order to relax enough to sleep.

Waking on the day that Tonks would be returning for her turn on guard duty sees Harry embarrassed at how eager he is to see her, his mind repeatedly returning to their encounter and is as aroused as he has ever been in his life.

Into this arrives Fleur Delacour who has been trying to reach Harry by owl mail ever since she returned to France after the Triwizard Tournament concluded. She's been having dreams about him, no, not dirty ones, per se, but she knows what it means when a Veela owes a wizard a life debt and she begins having repeated dreams where she dreams of said wizard and feels euphoria. She only has a certain amount of time to find Harry and make a formal bargain to settle her life debt to him, payment of money, oath of fealty, a formal promise of future aid or otherwise her own magic will treat the debt tying them together as though it was the start of a bond and eventually bind her to him.

It's not so much that Fleur finds Harry objectionable, only that she doesn't know him well enough, she's not in love with him. Scrying using the connection the debt forms between them gets her as far as Surrey before the wards around Harry confound her attempts to get a more specific location, which she solves by putting a tracking charm on an owl and sending Harry a letter. The mail wards ensure the owl won't find Harry but the owl circling confusedly around one particular street gets her as close as she needs to be to find Harry through the pull he is having on her magic.

Walking straight past a sleeping Dung, invisible under the hedge, she knocks on the door and is greeted by Harry, shirt unbuttoned, feverish and very aroused. Fleur is hit with a wave of desire that she can't resist, drawn powerfully to Harry and in moments has him on the lounge room couch, straddling him as she runs her hands over the lightly defined muscles of his wiry chest and flat stomach.

The two spent the next hour shagging each other's brains out, eventually with Tonks stumbling upon their activities as she arrives to relieve Dung and hears the moans, cries and shouts coming from within a house that should have a single occupant.

Shocked and aroused by what she finds Tonks finds herself watching as a voyeur, gradually and unconsciously removing her own clothing and pleasuring herself as she once again finds herself under the unknown spell she'd been subjected to two days earlier. Before she knows it she has joined Harry and Fleur and make it a threesome.

Once their lust is satisfied again Fleur is likewise shocked at what had just happened but finds herself, especially her Veela nature, immensely satisfied by what has occurred. She reflects as she watches the sleeping Harry and Tonks, that she should be upset that she has found herself in the exact situation she set out to avoid. Well, almost, she is bonded to Harry but now she can feel at least something similar linking herself to this woman whom she has never laid eyes upon before today.

Continuing On:
Tonks recruits Fleur into the Order and she joins the rotation on watching Harry, so she can remain close to her wizard, now that they've bonded. As time passes in each others company friendship and attraction beyond lust begins to develop between the three of them.

They continue their ongoing affair and it is during the one of their sessions that as Harry is ravaging Tonks that Fleur notices something, his fevered skin reveals slightly glowing runes down his spine and even in her lust addled state she manages to memorise a few.

Following that Tonks decides to take the runes Fleur writes down to a friend, a junior Unspeakable on attachment to the DMLE for when they encounter exotic magic, none other than Penelope Clearwater. Meeting Harry she feels the draw too, she can't get the runes to appear no matter what she tries and Fleur, amused by an idea, suggests that the next time Harry begins to experience the fever coming over him that she and Tonks can handle Harry while Penny, with appropriate precautions to prevent her being drawn into things, can observe and verify the other runes... maybe chained to a wall out of reach? Penny, blushing, suggests a viewing mirror.

Penelope confirms that it is a fertility spell of some kind and the fact the runes are manifesting out of Harry's magic and not carved onto his flesh suggests it's innate to him, not introduced. There's only a few ways that happens, a person's magic manifesting a spell without casting, a spell that they never put on themselves, and the most likely candidate is Family magic, a spell or ritual worked by an ancestor long ago and manifesting in their descendants.

Perhaps Penelope ends up entranced, perhaps not.

Hestia Jones accidentally stumbles into things when she volunteers, unknown to Tonks or Fleur, to cover for Dung when the scoundrel is sent by Dumbledore on a mission. She's rather more disconcerted by her tryst with Harry, she doesn't resent it but she doesn't feel comfortable with casual sex with a stranger and is somewhat unsettled to learn about the menage-a-trois going on between Harry, Fleur and Tonks already and has no intention of a multiple partner relationship.

Sirius eventually reveals the key facts they are missing when Harry is brought to Grimmauld Place after his birthday. There's a reason James was such a ladies man, a tom cat that chased skirts through fifth and sixth year even as he sought Lily's attentions. There's a fertility spell that passes down through the Potter family, cast long ago to reverse a sterility curse put on a male Potter who, at the time, like Harry, was the last male of the bloodline. It worked but resulted in a very extreme libido and his magic drew to him fertile women, any attraction they felt towards him was amplified the longer they were in proximity. The effect only ended once this ancestor of Harry's had produced several children with several women, satisfying the purpose of the original ritual, to ensure the continuance of the Potter bloodline.

When James, as an only child, began approaching adulthood the fertility ritual began to come into effect on him, as the last heir of the family and only his father remaining beside himself, he was compelled to further the bloodline and girls at the school who liked him were drawn to help. His marriage to Lily and her pregnancy had been the only things to lessen the effects but even then women had been known to make passes at him, much to Lily's mixed amusement and ire.

Harry as the only remaining Potter, period, and with a very powerful magical strength must be manifesting it early and more prominently. Maybe exerting his magic so much while duelling Voldemort, maybe Fleur's life debt beginning to form a bond between them, whatever the cause the ritual effect won't relent until the ritual recognises that the Potter bloodline is securely preserved.

Other Options:
Maybe Hermione, determined to check up on Harry when even her covert letters, sent through the muggle post, don't make it to him, arrives at Privet Drive and finds herself seduced. Perhaps it happens at Grimmauld Place when she begins snooping after becoming suspicious of Harry's periodic long absences which sometimes coincide with Tonks and/or Fleur and finds herself ensnared.

With each additional partner and regular relief Harry is less and less prone to drawing women in but at Hogwarts there will still be slip ups, especially with girls who need very little encouragement from the spell to make their own move on him. Given the ritual won't relent until Harry has at least one child and given that at their age most of the girls have no intention of becoming pregnant there's not much chance of the spell relenting any time soon.