Timeline:
Begins: Summer after Fifth Year.
Likely during or before Chapter 4 - 'An Excess of Phlegm'.
Ending: After Battle of Hogwarts or by preempting it.

Pairings:
Hermione/Ginny.
Harry/Hermione/Ginny.
Harry/Hermione/Ginny/Luna.
Harry/Hermione/Ginny/Luna/Tonks.
Harry/Harem (Large)
Extensive Femslash.

Story:
At the Burrow during the Summer before Sixth Year - and just before Harry is going to arrive in the company of Dumbledore - Hermione has already arrived and is sharing Ginny's bedroom. Witnessing each other in undressed states and Hermione asking Ginny's help to apply the potions and ointment to her wound (from Dolohov during the Battle at the Ministry) creates an escalating sexual tension that both try to ignore.

However the night before Harry's arrival Ginny awakes to the sound of Hermione quietly masturbating in the dark and moaning Harry's name when she comes. The next day the two talk about their mutual attraction to Harry and agree not to undermine each other, while both privately sure the other will catch Harry's attention and not them.

Over the next couple of days Harry, even as dense about girls as he is, realises both girls are flirting with him and is paralysed by indecision over how to respond and to which one. Hermione and Ginny talk about Harry and the looks each caught him giving the other, the whispers carrying on after lights out become far more naughty in speculating whether Harry is a breast or bum bloke and when Hermione accidentally discloses one of her fantasies about Harry Ginny reciprocates with her own.

In the dark they both masturbate while listening to the other's fantasy and get off immensely from the situation. Both avidly watch whenever Harry is paired off with the other when playing a game, relaxing or doing a chore. They both become more tactile and flirtatious with him leaving a very frustrated Harry waking repeatedly from unfinished erotic dreams about both of them.

Clothing is discarded entirely in the privacy of Ginny's room and mutual masturbation in the dark continues, however now they aren't imagining themselves with Harry while getting off but Harry with the other witch. Ginny comes powerfully while lewdly moaning Hermione's name, which is what finishes Hermione off.

Spooked when Hermione turns the light on and just looks at her Ginny starts to apologise before Hermione climbs atop of her and leans in to kiss her. Everything but penetration follows as the two girls thoroughly indulge the chance to explore, pleasure and get each other off. The lights back off afterwards Ginny suggests that all things considered, why don't they just share Harry?

They sneak up to Harry's room the next night when they think everyone is asleep. On waking Harry he accidentally believes they are part of his interrupted dream and gropes their legs and bums. His profuse apologies are interrupted by the explanation that they are both interested in him, romantically, and his conflicted distress as he looks from one to the other is quickly allayed when Hermione and Ginny snog each other before his very eyes.

A threesome lime follows as the girls' shirts come off and Harry's pants are pulled down. Each girl grinding herself to completion on Harry's thighs as the other two work them over before the Hermione and Ginny give him a double handjob.

Days follow with Hermione and Ginny covering for the other with the rest of the Weasleys while one of them sneaks off somewhere to snog and fondle with Harry. One night Harry sneaks down to visit Ginny and Hermione in their room and gives them both a tongue-lashing but barely escapes through the window on Ginny's broom when Molly comes down the stairs.

While all three have reservations about how quickly things are moving they are ill-equipped to slow things down as each one is goaded on by both the others in the heat of the moment. They still manage to abstain from going all the way, at least while at the Burrow. Trysts in the broom shed, the kitchen table early in the morning, out in the orchard, in the bathroom.

One afternoon Ginny, Hermione and Harry convince Molly to let them go mushroom hunting down near the local pond on the edge of the Weasley property. Their skinny dipping is interrupted by the realisation they have an audience in the form of Luna who is already naked and walking into the water when they notice her, with a polite 'Good afternoon' from the quirky blond.

Ginny has had thoughts about Luna before and is entranced while Harry tries his best not to look, right up until Luna invites him to look all he likes and not to let her interrupt. After a short hesitation Ginny and Harry go back to work driving Hermione wild (who is even more aroused by the exhibitionism), then Luna comes closer to observe Hermione and Ginny's fellatio technique before remarking she has neglected herself and then pleasures herself to completion as the other three watch.

Luna is soon visiting the Burrow and aiding in the scheme to give the other two time for fun and games with Harry and even gets the chance to watch Ginny and Hermione together. Hermione and Ginny quickly come to the agreement that the possibility of Luna fully joining in would be completely acceptable by them and shortly after a pole-axed Harry is stunned to be left with Hermione while she runs interference so Ginny and Luna can sneak away.

One morning while coming down the stairs Harry collides with Tonks who stayed over the night before in Percy's old room... and is dressed only in a towel, after accidentally leaving her wand in the bedroom. Harry ends up with a face full of her fantastic breasts when she lands on top of him. Hermione hearing the collision comes up the stairs and finds the scene, stunning Tonks by staring lasciviously at the Metamorph's exposed body (before she covers herself up) and helping Harry up, glancing at Harry's largely tented pants and smirking a reassurance to Tonks that she'll 'take care of Harry'.

Now clued in Tonks watches carefully and is stupefied when she doesn't witness Harry and Hermione engaging in shenanigans but rather Harry and Ginny and then Harry and Luna (after Hermione and Ginny wink at him as Luna drags him away). After witnessing all this Tonks sees Fleur smirking and the Frenchwoman merely remarks 'Quite the Cassanova, non?'.

Shortly after it's Ginny's birthday and an excellent party with good food, Order members and Fred and George for entertainment follows. However it's the after-party that interests Harry and the girls and after it's over they sneak off to go back to the pond for some fun - while Ron is stuck at the Burrow doing his homework. Tonks feigns that she has to go to the Ministry and slips away but instead follows Harry and company and witnesses the following debauchery with wide-eyes and yet is unable to look away.

Afterwards she apparates home and shamefully uses some of her toys to get herself off while thinking about the rutting orgy of teenagers she just spent an hour watching.

Further Plot Ideas:
Between Ginny's Birthday (August 11th) and the Hogwarts Express there's the visit to Diagon Alley. Tonks would necessarily be part of that detail and what would she think was going on when Harry and Hermione (and Luna or Ginny) disappear under the Cloak to go follow Malfoy? Confronting them afterwards and outing the fact she knows about their fun and games, how irresponsible it was to duck their protection to have a shag. Luna might ask if that means that next time Tonks wants to be able to see them when they are shagging. Was there anyone else at the Alley who spotted Harry, Hermione, Ginny and Luna doing anything with each other? Some classmate perhaps, like Katie Bell while coming to drop in on Angelina at Fred and George's shop? Susan and Hannah perhaps?

Harry gets ambushed by Draco on the Express, as in canon, and it is once again Tonks who finds him petrified and under his Cloak. 'Harry Potter, completely at my mercy... I should ransom you back to the girls!'. 'Oh... in exchange for what?' Harry asks cheekily. 'Uh... nevermind' Tonks mumbled, her cheeks darkening despite her best efforts.

The four wait until they can get to Hogwarts so they can covertly brew the necessary contraceptive potions and have the necessary privacy (and versatility) of the Room of Requirement before they fully lose their virginities to each other. Meanwhile Luna provides some sex manuals from her family's library - there's a reason they are called the Lovegoods, before ritual magic fell out of favour they were practitioners of sex and fertility magic. The combination of rare magical knowledge in old tomes and the intensely erotic nature of the material is catnip to Hermione's inclinations and she devours the books and learns all sorts of interesting spells and rituals that three virgin witches could work with the appropriate preparations before their deflowering.

What is the spell? How does it affect Harry or all of them? Increased magical power or sharing power? Increased sexual stamina or even a sexual aura? Specific magical talents, i.e. Mage Sight, some form of Telepathy minor or major, an Elemental gift/talent? Take your pick.

Tonks is undercover in the school body, posing as a Fifth/Sixth/Seventh year who was home-schooled until her parents decided she'd be safer from Death Eaters at Hogwarts for the coming year. When Katie asks Harry if she's keeping her position as Chaser on the Quidditch team his silence makes her nervously laugh and suggest that 'I can be very persuasive you know, Harry'. Harry instead turns the Captaincy over to her, he'll be too busy (restarting the DA)... and too 'busy' as well. Now it's Harry's turn to persuade her, she insists flirtatiously. With the DA reformed Susan, Hannah, Padma, Parvati, Lavender, Cho and other witches start to observe Harry's interactions with Hermione, Ginny and Luna and in their gossip can't agree on which they think is with him... and later when some of them (somehow) learn the truth not a few of the DA girls become even MORE interested in Harry.

It would be best if the lemons were mostly erotic rather than porn-smut and no neglecting of the girl-girl links in the relationship in favour of just Harry constantly scoring in everything. Another sex-ritual once yet more girls begin joining the ranks?